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MODERN    MICROSCOPY 


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A   Typical   Modern    Microscope. — Made    by   "W.    Watson   and   Sons,    Ltd..    to    the 
Specification    of    the    late    Dr.    Henri   Van    Heurck,    Antwerp,    for    Photo- 

MlCROGRAFHIC   AND   HlGH-PoWER   WORK. 


Modern  Microscopy 


H  1E)anC>boo^  for  Beginners  anfc  Students 


BY 

M.    I.    CROSS 

AND 

MARTIN    J.    COLE 

LECTURER   IN   HISTOLOGY   AT   COOKE'S   SCHOOL   OF   ANATOMY 


FOURTH   EDITION 

REVISED    AND    ENLARGED 

WITH  CHAPTERS  ON  SPECIAL  SUBJECTS  BY  VARIOUS  WRITERS 


I      ( 


CHICAGO 
CHICAGO    MEDICAL    BOOK    COMPANY 

1912 


PREFACE   TO   FOURTH   EDITION 

In  the  preparation  of  this  new  edition,  the  original  intention 
that  it  should  be  for  beginners  and  students  has  been  steadily 
kept  in  view.  Numerous  additions  and  revisions  have  been 
made  in  order  to  bring  the  book  into  line  with  present-day 
knowledge  and  methods. 

Through  the  kindness  of  specialists  in  their  own  depart- 
ments, a  third  part  has  been  added,  giving  information  on 
many  subjects  in  which  amateur  microscopists  in  particular 
are  interested.  To  these  contributors,  and  to  numerous  other 
friends  who  have  assisted  by  advice,  suggestions,  and  correcting 
proofs,  thanks  are  most  cordially  given. 


M.  I.  C. 


London, 


November,  1911. 


1 


1C73 


Vll 


: 


PREFACE   TO   FIRST   EDITION 


This  handbook  is  not  intended  to  be  an  exhaustive  treatise  on 
the  microscope,  nor  to  give  particulars  of  the  various  patterns  of 
instruments  that  are  made,  of  which  details  can  be  seen  in  the 
makers'  catalogues,  but  to  afford  such  information  and  advice 
as  will  assist  the  novice  in  choosing  his  microscope  and  acces- 
sories, and  direct  him  in  his  initial  acquaintance  with  the  way 
to  use  it. 

The  directions  for  preparing  microscopic  objects  by  Mr.  Martin 
J.  Cole  are  the  outcome  of  a  very  long  experience  as  a  preparer 
of  Microscopic  Objects  of  the  highest  class,  and  cannot  fail  to  be 
of  the  greatest  service  to  the  working  microscopist. 

M.  I.  CROSS. 


Mil 


CONTENTS 


PART  I 

PAGES 

Introduction   --------        1-4 

CHAPTER  I 

THE    MICROSCOPE-STAND 

On  choosing  a  microscope- stand — Designation  of  the  parts  of  a  micro- 
scope—  Different  forms  of  feet  compared  —  The  stage — Sub-stage 
and  under-fitting — Coarse  and  fine  adjustments — Two-speed  fine 
adjustments — The  limb — The  body-tube — The  mechanical  draw- 
tube — Tailpiece  and  mirrors — Binocular  microscopes — Microscopes 
for  special  purposes — Dissecting  microscopes — Magnifiers  for  dis- 
secting— Microscopes  for  metallurgy — Microscopes  for  travellers — 
Microscopes  for  petrology  and  for  preparing  and  mounting — The 
ultra-microscope     -  ...       5-49 

CHAPTER  II 

OPTICAL    CONSTRUCTION 

Optical  definitions  —  Magnifying  power,  how  produced  —  Objectives, 
apochromatic  and  achromatic  —  Immersion  objectives  —  Angular 
and  numerical  apertures — Measuring  apertures  with  apertometers — 
Influence  of  the  cover-glass  and  correction  for  various  thicknesses 
— The  use  of  the  correction  collar — Testing  objectives — The  Abbe 
test-plate — Choice  of  objectives — How  a  microscope  objective  is 
made — Eyepieces:  Huyghenian,  compensating,  Holoscopic,  Kellner, 
projection  and  binocular — Standard  gauges  for  eyepieces  -     50-88 

CHAPTER  III 

ILLUMINATION    AND    ILLUMINATING    APPARATUS 

Monochromatic  light  and  light ]  filters — Sub-stage  condensers — Aplan- 
atic  apertures  of  condensers — How  to  use  the  condenser — Choice 
of   a   condenser — Dark-ground   illumination   by   immersion   para- 

ix 


x  CONTENTS 

PAGES 

boloids — Spot  lens — Indian  ink  method — The  polariscope — Selenites 
— Selenite  stages — The  bull's-eye  condenser — Parabolic  reflector — 
The  vertical  illuminator  ------     89-115 

CHAPTEE  IV 

ACCESSORY    APPARATUS 

Lamps  :  Electric,  incandescent  gas  —  Objective  changers  —  Davis's 
shutter — Drawing  apparatus— Measurement  of  objects — Troughs 
—  Compressors — Stage  forceps — Eye-shade — Hints  regarding  the 
care  of  the  microscope — Text-books — Choice  of  an  outfit  -     116-133 

CHAPTEE  V 

A  SHORT  NOTE  CONCERNING  THE  INFLUENCE  OF  DIFFRACTION  ON  THE 
RESOLVING  POWER  OF  MICROSCOPICAL  OBJECTIVES,  AND  ON  THE 
APPARENT  COLOUR  OF  MICROSCOPICAL  OBJECTS,  BY  THE  LATE  DR.  G. 
JOHNSTONE    STONEY,  F.R.S.  -----       134-136 


PAET  II 

CHAPTEE  VI 
Introduction  ._-_.._  137 

List  of  tissues  and  organs,  and  the  most  suitable  hardening,  staining, 

and  mounting  reagents  -  138-141 

CHAPTEE  VII 

HARDENING   AND    PRESERVING   ANIMAL    TISSUES    AND    ORGANS    FOR 
MICROSCOPICAL    EXAMINATIONS 

Absolute  alcohol — Chromic  acid  and  spirit — Potassium  bichromate — 
Ammonium  bichromate — Miiller's  fluid — Methylated  spirit — De- 
calcifying solution  —  Olfactory  region — Cochlea — Corrosive  subli- 
mate —  Picric  acid  —  Formaldehyde  —  General  directions  for 
hardening  tissues  .._._.     142-145 

CHAPTEE  VIII 

EMBEDDING   TISSUES    AND    SECTION-CUTTING 

Cutting  sections  with  a  razor  by  hand — Embedding  in  paraffin  wax 
and  lard — Infiltrating  a  tissue  with  paraffin — Embedding  bath — 
Cole's  microtome  and  embedding  in  carrot — Cathcart's  microtome 
for  freezing — Embedding  in  celloidin — Embedding  in  gelatine — 
Cambridge  rocking  microtome  -----     146-153 


CONTENTS  xi 

CHAPTER  IX 

P  \ORS 

STAINING    ANIMAL    SECTIONS    AND    MOUNTING    IN    CANADA    BALSAM 

Grenadier's  alcoholic  borax  carmine — Staining  process — Staining  with 
Ehrlicji's  hematoxylin — Double  staining  with  hematoxylin  and 
eosin — Aniline  blue-black — Aniline  blue — Golgi's  nitrate  of  silver 
methods  —  Mounting  in  Canada  balsam  —  Staining  in  bulk — 
Flemming's  method  for  staining  nuclei — Weigert-Pal  method  of 
staining — Ehrlich's  triple  stain  for  blood-corpuscles — Toison's  stain 
for  white  blood-corpuscles — Fixing  and  staining  sections  on  the 
slide — Mayer's  albumen  method — Shellac  method        -  -     154-161 

CHAPTER  X 

STAINING    BLOOD    AND    EPITHELIUM,    TEASING-OUT    TISSUES 

AND    MOUNTING    IN    AQUEOUS    MEDIA — STAINING    WITH    PICROCARMINE, 

GOLD    CHLORIDE,    SILVER    NITRATE,    AND    OSMIC    ACID 

Double  staining  nucleated  blood-corpuscles — Blood  of  mammals,  non- 
nucleated  corpuscles  —  Epithelium  —  Endothelium  —  Teasing-out 
tissues  —  Staining  with  picrocarmine  —  Farrant's  medium  — 
Glycerine  jelly    -------     162-166 

CHAPTER  XI 

STAINING    AND    MOUNTING   MICRO-ORGANISMS 

Staining  bacteria  on  cover-glasses  —  Ehrlich's  method  for  double 
staining — Bacillus  tuberculosis — Ziehl  Neelsen's  method— Gibbe's 
double  stain — Anthrax  bacillus — Loffler's  alkaline  blue  method — 
Gram's  method — Leprosy  bacillus — Diphtheria  bacillus— Glanders 
bacillus — Kuhne's  method — Schutz's  method — Syphilis  bacillus — 
Bacillus  of  enteric  fever  —  Gaffky's  method  —  Spirillum — Actino- 
mycosis— Weigert's  method — Haematozoa  of  Laveran  —  Filaria — 
Vermes — Anchylostoma — Trichina  spiralis       -  -  -     167-178 

CHAPTER  XII 

INJECTION    OF    BLOODVESSELS 

Carmine  and  gelatine  injecting  mass — Prussian  or  Berlin  blue  and 
gelatine  mass — Watery  solution  of  Berlin  blue — Injecting  appa- 
ratus— Directions  for  injecting — Injecting  lymphatics — Injecting 
lymph-sinuses  of  glands  ....  -     174-176 

CHAPTER  XIII 

CUTTING,    STAINING,    AND    MOUNTING    VEGETABLE    SECTIONS 

Ovaries — Anthers — Section-cutting — Bleaching- —  Staining  with  borax 
carmine — Haematoxylin — Double  staining — Dalton- Smith's  method 


xii  CONTENTS 

PAGES 

— Cole's  method — Staining  with  eosin — Staining  hairs  on  leaves 
— Fucus  and  other  algae  —  Ovaries  of  flowers  —  Flower-buds — 
Pollens — Pharmacological  specimens — Powdered  drugs — Mounting 
in  Canada  balsam  .-__..     177-184 

CHAPTER  XIV 

THE    PREPARATION    OF    VEGETABLE    TISSUES    FOR    MOUNTING    IN    GLYCERINE 
JELLY,    ACETATE    OF    COPPER    SOLUTION,    ETC. 

Epidermis  for  stomata — Staining  the  epidermis — Annular  vessels — 
Scalariform  vessels — Spiral  vessels — Raphides — Starches — Yeast 
— Mycetozoa  and  fungi — Preserving  fluid  for  green  algae — Proto- 
coccus  —  Antheridia  and  archegonia  of  mosses  —  Fertile  branch 
of  chara — Prothallus  of  fern — Sporangia  and  spores  of  fern — Iso- 
lating antheridia  and  oogonia  from  fucus — Digestive  glands  in 
pitcher  plant  —  Aleurone  —  Marine  algae  —  Mounting  process  — 
Corallines  .......     185-193 

CHAPTER  XV 

CUTTING,    GRINDING,    AND    MOUNTING    SECTIONS    OF    HARD    TISSUES — 

PREPARING    METAL    SPECIMENS 

Bone — Rock    sections — Metal    specimens— Etching — Ferrite — Cemen- 

tite— Pearlite      -------     194-197 

CHAPTER  XVI 

PREPARING    AND    MOUNTING    ENTOMOLOGICAL    SPECIMENS    FOR    THE 

MICROSCOPE 

Preparing  whole  insects  for  mounting  with  pressure  in  Canada  balsam 
— Mounting  insects  in  Canada  balsam  without  pressure — Mount- 
ing insects  in  glycerine  without  pressure  -  -  -     198-203 

CHAPTER  XVII 

CRYSTALS    AND    POLARISCOPE    OBJECTS 

Crystals — Crystals  of  silver  —  Starches  —  Sections  of  starch-bearing 
tissues — Cuticles  containing  raphides — Cuticles  of  leaves — Cotton, 
hemp,  wool,  silk,  flax,  etc. — Scales  of  leaves — fish  scales — Palates 
— Sections  of  hairs  and  quills — Small  fine  hairs — Horns,  hoofs, 
whalebone  and  claws— Decalcified  bones — muscular  fibres       -     204-208 

CHAPTER  XVIII 

CLEANING    AND     MOUNTING    DIATOMS,     POLYCYSTINA,     AND 

FORAMINIFERA 

Cleaning  diatoms  growing  upon  algae  or  shells — To  clean  fossil  diato- 
maceous  deposits — To  clean  living  diatoms— To  clean  polycystina 


CONTENTS  xiii 

PAGES 

— To  clean  foraniinifera — Mounting  diatoms  in  Canada  balsam — 
Unselected  polycystina,  transparent  and  opaque — Selected  diatoms 
and  polycystina — Foraminifera,  unselected  transparent  and  opaque 
mounts — Spicules  of  Gorgonia — Spicules  of  Alcionium — Sponges 
to  show  cell  structures — The  skeleton — Sections  of  sponges     -     209-214 

CHAPTEK  XIX 

DRY   MOUNTS 

Opaque  cells — Transparent  cells — Opaque  mounts  of  pollens  -     215-216 

CHAPTER  XX 

FINISHING    OFF    SLIDES 

Canada  balsam — Quick  method — Exposure  method — Glycerine  jelly — 

Farrant's  medium — Dry  mounts — Cleaning  off  failures  -     217-218 


PAKT  III 
CHAPTER  XXI 

AN   INTRODUCTION   TO    THE    USE    OF   THE    PETROLOGICAL    MICROSCOPE, 
BY    FREDERIC    J.    CHESHIRE,    F.R.M.S. 

Polarization  of  light — Polarized  light — Double  refraction — Newton's 
colour  scale  according  to  Quincke — Rotary  polarization — Optical 
adjuncts  for  the  petrological  microscope  —  Mica  quarter-wave 
plate — Klein  quartz  plate — Bertrand  plate — Fedorow  mica-steps — 
Quartz  wedge  —  Construction  of  the  petrological  microscope  — 
Modern  petrological  microscope — Preliminary  adjustment  of  petro- 
logical microscope — Centring  of  the  objective — Rectangularity  of 
the  cross- wires  in  the  eyepiece — Rectangularity  of  the  vibration 
planes  of  the  Nicol  prisms — Parallelism  of  the  cross-wires  to  the 
vibration  planes  of  the  crossed  nicols — Examination  and  identifi- 
cation of  the  crystalline  constituents  of  rock  sections    -  -     219-236 


CHAPTER  XXII 

ROTIFERA,    BY    C.    F.    ROUSSELET,    F.R.M.S. 

."Specimens  likely  to  be  found  during  each  month  of  the  year — Collect- 
ing-grounds near  London — Methods  of  collecting,  preliminary 
examination,  and  keeping — Apparatus  for  microscopic  examina- 
tion— Preserving  and  mounting  -  237-262 


xiv  CONTENTS 

CHAPTER  XXIII 

PAGES 

THE    COLLECTION,    EXAMINATION,    AND    PRESERVATION    OF    MITES 
FOUND    IN    FRESH    WATER,    BY    C.    D.    SOAR,    F.L.S.,    F.R.M.S. 

Instructions   for   collecting,    examining,    preserving,    and   mounting — 

Text-books  -------     263-269 

CHAPTER  XXIV 

COLLECTING  AND  PREPARING  FORAMINIFERA,  BY  ARTHUR 

EARLAND,  F.R.M.S. 

Sources  for  foraminifera — Treatment  of  foraminifera — The  use  of  the 
dredge — The  shore  collector — Cleaning  and  mounting  methods — 
Microscopical  examination  and  mounting — Artificial  casts — Cutting 
sections  of  foraminifera  -  270-281 

CHAPTER  XXY 

NOTES    ON    THE    COLLECTION,    EXAMINATION,    AND    MOUNTING    OF    MOSSES 
AND    LIVERWORTS,    BY    T.    H.    RUSSELL,    F.L.S. 

Collection  of  specimens — Preparation — Mounting  materials  :  glycerine 

jelly;  formalin;  varnishes  -  282-291 

CHAPTER  XXYI 

THE    MICROSCOPE    AND    NATURE    STUDY,    BY    WILFRED    MARK 

WEBB,    F.L.S. 

Suggested  subjects  in  which  the  microscope  may  be  employed        -     292  -298 

CHAPTER  XXYII 

THE    MICROSCOPY    OF    FOODS,    BY    CUTHBERT    ANDREWS,    F.R.M.S. 

Starches  —  Butter  —  Cheese  —  Milk  — Water — Tea  —  Coffee  —  Cocoa — 
Mustard — Pepper — Sugar — Flesh  foods  —  Typical  tissues — Re- 
agents for  the  work        ------     299-318 

Index  -.--_._.     319-325 


LIST  OF  ILLUSTRATIONS 


FIG. 


PAGE 


A  Typical  Modern  Microscope  -  -  -     Frontispiece 

1.  A  Typical  Student's  Microscope  (Watson)     -             -             -  7 

2.  D.P.H.  Microscope  (C.  Baker)            .....  9 

3.  Section  through  Microscope,  Stand  III.  (Zeiss)          -             -  -12 

4.  Attachable  Mechanical  Stage  (Zeiss)               -             -             -  -  15 

5.  Eesearch  Microscope,  Stand  BB  (Bausch  and  Lomb)            -  -  17 

6.  Diagonal  Hackwork  and  Pinion          -             -             -             -  -  21 

7.  Sectional  View  of  Adjustable  Fittings  of  Rack  and  Pinion  -  -  21 

8.  Microscope  with  Modified  Horseshoe  Foot  (Leitz)    -             -  -  23 

9.  Construction  of  Lever  Fine  Adjustment        -             -             -  -  24 

10.  '  Imperial  Microscope  '  (E.  and  J.  Beck)        -             -             -  -  25 

11.  Milled  Head  for  giving  Two  Speeds  to  Fine  Adjustment      -  -  27 

12.  Sectional  View  of  Body  of  Microscope           -             -             -  -  29 

13.  Sectional  View  of  Body  of  Wenham  Binocular  Microscope  -  -  34 

14.  Dissecting  Microscope,  Simple  Pattern          -             -             -  -  36 

15.  Monocular  Prism  Dissecting  Microscope  (Zeiss)         -             -  -  38 

16.  Aplanatic  Magnifier  -             -             -             -             -             -  -  39 

17.  Metallurgical  Microscope  (Watson)    -             -             -             -  -  40 

18.  Holder  for  Metallurgical  Work           -             -             -             -  -  41 

19.  Traveller's  Microscope  (Swift  and  Son)          -             -             -  -  42 

20.  Penological  Microscope  (Swift  and  Son)        -             -             -  -  45 

21.  Inexpensive  Microscope  (Leitz)          -             -             -             -  -  46 

22.  Diagram  showing  Bays  received  by  Dry,  Water  Immersion,  and  Oil 

Immersion  Objectives          -             -             -             -             -  -  60 

23.  Abbe's  Apertometer    -                          -             -             -             -  -  60 

24.  Cheshire's  Apertometer           -             -             -             -             -  -  66 

25.  Correction  Collar  on  Objective            -             -             -             -  -  70 

26.  Abbe  Test-Plate           -------  75 

27.  Tools  used  for  Microscope  Objective  Making              -             -  -80 

28.  Proof  Plate  and  Newton's  Eings        -             -             -             -  -  82 

29.  Huyghenian  Eyepiece             -  83 

30.  Holoscopic  Eyepiece  -------  85 

31.  Projection  Eyepiece    -             -             -             .             -             -  -  86 

32.  Binocular  Eyepiece  (Zeiss)     -             -             -             -             -  -  88 

33.  Abbe  Illuminator,  Optical  Part          -             -             -             -  -  94 

34.  Achromatic  Condenser  (C.  Baker)     -             -             -             -  -  95 

xv 


xvi  LIST  OF  ILLUSTKATIONS 


PAGE 


-Apertures  of  Condensers      -----  98 


35.  Iris  Diaphragm           -             -             -             -             -             -             -  96 

36. 
37. 
38. 
39. 

40.  Image  of  Lamp-Flame           ------  100 

41.  Back  Lens  of  Objective           ------  102 

42.  Stops  for  Condensers  -----                          -  103 

43.  Diagram  of  Eays  passing  through  Immersion  Paraboloid     -             -  106 

44.  Polarizer                       -             -             -             -             -             -             -  108 

45.  Analyzer         --------  108 

46.  Darker's  Selenites       -----                          -  109 

47.  Mica-Selenite  Stage   -------  110 

48.  Bull's-Eye  Condenser  -  -  -  -  -  -111 

49.  Diagrams  of  Effects  with  Bull's-Eye  Condenser        -             -             -  112 

50.  Vertical  Illuminator  -             -             -             -             -             -             -  114 

51.  Microscope  Lamp  with  Metal  Chimney         -             -             -             -  117 

52.  Dustproof  Triple  Nosepiece    -             -             -             -             -             -  118 

53.  Sliding  Objective  Changer,  by  Zeiss  -             -             -             -             -  119 

54.  Davis's  Shutter            -             -             -             -             -             -             -  120 

55.  Abbe  Camera  Lucida               ------  122 

56.  Jackson  Eyepiece  Micrometer            -  125 

57.  Ramsden  Screw  Micrometer               -             -             -             -             -  125 

58.  Botterill's  Trough       -                                                                                -  126 

59.  Rousselet's  Live-Cage             .--___  127 

60.  Rousselet's  Compressorium    ------  128 

61.  Eye-Shade  for  Monocular  Microscope            -  128 

62.  Cheap  Embedding  Bath          -             -             -             -             -             -  147 

63.  Cole's  Pattern  Microtome       -             -             -             -             -             -  148 

64.  Cathcart's  Microtome              -             -             -             -             -             -  150 

65.  Rocking  Microtome  (Cambridge  Scientific  Instrument  Company)    -  153 

66.  Rope  Polariscope,  Nicols  parallel       -----  220 

67.  Rope  Polariscope,  Nicols  crossed       -----  221 

68.  Crystals  of  Quartz       -------  223 

69.  Quartz  cut  Parallel  to  Axis    ------  224 

70.  Quartz  cut  Normal  to  Axis,  to  show  Radial  Polarization      -             -  226 

71.  Bertrand  Plate            -             -             -             -             -             -             -  228 

72.  Mica-Steps      --------  228 

73.  Quartz  Wedge             -             -             -             -             -             -             -  229 

74.  Modern  Petrological  Microscope  (Leitz)         -  230 

75.  The  Action  of  a  Convergent  System               -  234 

76.  Diagram  of  Arrangement  for  Convergent  Light        -             -             -  235 


Interference  Figures  of  Arragonite.    Plate  I.  -     Facing  page     236 


LIST  OF  ILLUSTEATIONS 


xvii 


PAGE 

246 
252 
253 
264 
265 

266 


Plate  II.      Facing  page     296 


Plate  III.    Facing  page     300 


81.  Collecting-Net  ---.-._ 

82.  Aquarium  Microscope  -  -  -  - 

83.  Apparatus  used  for  Pond-Life 

84.  Larva,  Piona  Longipalpis     ------ 

85.  Larva,  Nymph,  and  Adult,  Hydrachna  Globosa 

86.  Larvse    of    Hydrachnid    Parasite    on    Dytiscus    Marginalis   and 

Nepa  Cinerea        ------- 

87.  Section  of  Flower-Bud  of  Lily 

88.  Lophopus  Crystallinus 

89.  Stephanoceros  Eichhornii    - 

90.  Starch  of  Wheat 

91.  Starch  of  Rye 

92.  Starch  of  Barley 

93.  Film  of  Butter 

94.  Penicillium  Glaucum 

95.  Cheese  Mites 

96.  Film  of  Milk 

97.  English  Diatoms 

98.  Sewage  Fungus 

99.  Leaf  of  Tea 

100.  Section  of  Coffee-Berry 

101.  Typical  Cells  of  Coffee 

102.  Head  of  Cysticercus 

103.  Trichina  Spiralis 

104.  Oxyuris  Vermicularis 

105.  Liver  Fluke  of  Sheep 

106.  Bacillus  Anthracis    - 

107.  Human  Blood- Discs 

108.  Cocoa  -  .  .  . 

109.  Mustard        - 

110.  Section  of  Peppercorn 

111.  Pepper  - 

112.  Cane-Sugar  - 

113.  Segment  of  Taenia  Solium    - 


Plate  IV.     Facing  page     306 


>    Plate  V.       Facing  page     310 


►    Plate  VI.     Facing  page     314 


MODERN  MICROSCOPY 


INTRODUCTION 

We  have  in  this  book  to  do  with  the  microscope  of  to-day,  and 
the  history  of  its  development,  interesting  as  it  must  be,  cannot 
be  traced  here.  Since  the  first  edition  of  this  book  was  written, 
eighteen  years  ago,  the  microscope  has  become  increasingly  a 
necessity  of  civilized  life.  The  vast  number  of  discoveries  con- 
nected with  bacteriology,  diseases  of  the  blood  and  tissues,  and 
almost  everything  that  affects  our  well-being,  have  been,  and 
are,  due  to  investigations  and  observations  which  are  made  by 
the  aid  of  the  microscope.  There  is,  consequently,  a  rapidly  in- 
creasing class  of  professional  workers  ;  and  whereas  in  days  gone 
by  the  amateur  was  the  principal  user  of  the  microscope,  he  has 
been  far  outstripped  by  those  to  whom  the  instrument  is  a  vital 
necessity.  Hundreds  of  microscopes  are  now  manufactured  to 
the  one  of  years  ago.  Naturally,  instruments  have  been  designed 
to  suit  the  special  needs  of  the  army  of  workers,  and  the  old 
order  of  things  has  been  changed.  With  the  increasing  demand, 
keen  emulation  amongst  manufacturers  has  resulted  in  improve- 
ments in  both  optical  and  mechanical  arrangements,  and  the 
so-called  students'  series  of  objectives  by  the  leading  makers  of 
to-day,  at  nominal  prices,  are  in  many  instances  superior  to  the 
most  expensive  lenses  of  former  days.  Those  who  are  interested 
in  the  scientific  and  artistic  aspects  of  microscopy  can  view  with 
nothing  but  satisfaction  the  distinctly  progressive  nature  of 
everything  connected  with  the  instrument  itself  and  the  absorb- 
ing secrets  it  reveals.  That  the  onward  march  will  continue  is 
certain,  for  the  issues  which  depend  on  the  microscope  for  their 


N  CState  ColW» 


2  MODERN  MICROSCOPY 

solution  are  ever  increasing  in  number.  The  demands  which 
are  made  on  it  are  constantly  growing  in  exactness  and  variety, 
and  there  are  always  ready  minds  and  willing  hands  to  devise 
facilities  for  meeting  them.  While  it  is  true  that  the  increasing 
use  of  the  microscope  has  been  an  important  element  in  the 
improvements  that  have  taken  place,  the  fact  cannot  be  over- 
looked that  much  has  been  due  to  the  interest,  unceasing 
criticism,  impartial  examination,  and  well-merited  recommenda- 
tion, together  with  the  suggestion  fraught  of  knowledge,  skill, 
and  thoughtful  consideration  on  the  part  of  expert  amateurs, 
many  of  whose  names  are  familiar  to  every  worker  with  the 
microscope. 

To  the  microscopical  societies  also  the  evolution  of  the 
microscope  is  due  in  no  small  degree,  and  especially  does  this 
apply  to  the  Royal  Microscopical  Society  and  the  Quekett 
Microscopical  Club,  both  of  which  meet  in  London  at  20,  Hanover 
Square.  Every  improvement  in  the  instrument  and  its  acces- 
sories that  takes  place  is  presented  to  these  societies  for  criticism 
and  in  connection  with  both  of  them,  as  officers  and  membeis, 
are  men  who  have  attained  the  highest  eminence  in  microscopical 
science  and  manipulation,  whose  judgments  have  influenced  and 
moulded  the  character  of  microscopy,  and  who  are  ever  willing 
to  assist  by  advice  and  suggestion  any  who  will  avail  themselves 
of  their  experience.  It  is  most  desirable  that  microscopists 
should  become  members  of  a  good  microscopical  society,  and 
those  just  mentioned  enjoy  the  highest  position  in  England. 

It  is  surprising  that,  notwithstanding  the  pleasures  and 
advantages  that  are  associated  with  microscopical  work,  micros- 
copy has  not  the  hold  on  people  of  refined  tastes  that  its  merits 
should  fairly  claim  for  it.  It  does  not  seem  to  be  realized  that 
the  microscope  will  unfold  its  wonders  and  beauties  without  that 
long  and  careful  study  which  is  the  necessary  preliminary  to  the 
majority  of  scientific  pursuits.  Those  who  may  be  induced  to 
use  a  microscope  in  the  first  place  for  pleasure  and  recreation 
will  quickly  find  their  inclinations  leading  them  to  a  desire  for 
fuller  knowledge  concerning  the  subjects  which  may  come  under 
their  notice,  and  by  degrees  this  instrument  will  become  the 
means  of  the  acquirement  of  a  very  liberal  education,  for  its 
influence  is  not  merely  confined  to  one  kingdom  ;  it  embraces 


INTKODUCTION  3 

every  tangible  and  intangible  subject,  whether  it  be  the  air  we 
breathe,  with  its  myriads  of  invisible  friends  and  foes  to  human 
well-being,  or  the  floors  of  oceans,  with  their  minute  flinty  shells 
bearing  markings  which  exceed  in  accuracy  the  power  of  any 
draughtsman  to  depict,  and  which  in  themselves  are  invisible 
to  the  naked  eye,  many  of  them  measuring  but  the  j^Vo  of  an 
inch. 

Astronomy,  with  all  the  wonders  that  are  associated  with  the 
study,  demands  many  a  night  vigil,  an  expensive  instrument, 
and  a  suitable  observatory,  and  even  then  there  is  always  a 
sense  of  dissatisfaction  when  accounts  are  read  of  observations 
of  fellow-workers  who  are  more  highly  favoured  in  that  they 
possess  glasses  of  very  large  aperture,  which  the  average 
astronomical  student  could  not  aspire  to.  How  dependent  the 
observer  is  on  the  weather,  too  ! 

Photography,  with  its  manifold  uses  and  the  pleasant  memories 
associated  with  numerous  pictures  that  are  secured,  cannot  be 
compared  with  the  microscope,  more  especially  for  the  long 
winter  evenings. 

For  any  kind  of  recreation  to  produce  the  mental  rest  which 
is  required  by  the  man  of  business,  a  fresh  set  of  faculties  must 
be  brought  into  play,  and  no  better  method  can  be  imagined  for 
the  purpose  than  the  introduction  to  a  world  whose  variety  is 
surprising  and  illimitable,  whose  form  is  lovely  and  unique,  and 
whose  subjects  can  never  be  met  with  excepting  in  the  quiet 
observations  through  the  microscope-tube  in  the  study. 

Perhaps  some  people  may  hesitate  to  attempt  working  with 
the  microscope,  not  caring  to  use  it  merely  as  a  means  of  amuse- 
ment, and  mistrusting  their  ability  to  employ  it  scientifically. 
They  reflect  that  every  department  has  its  untiring,  experienced 
workers,  and  available  ground  appears  to  have  been  gone  over 
so  repeatedly  that  it  would  seem  hopeless  for  an  amateur  to 
attempt  to  add  to  existing  knowledge  on  any  subject.  This  idea 
is  a  mistaken  one,  and  any  microscopist  who  uses  his  instrument 
thoughtfully  will  be  surprised  at  the  manner  in  which  the  love 
for  the  work  will  grow  upon  him,  and  how  gradually  he  will 
become  master  of  some  special  department  which  he  has  adopted 
as  his  own.  On  this  point  we  would  echo  the  words  of  a  well- 
known  microscopist :  '  It  needs  no  marvellous  intellect,  no  special 


4  MODEKN  MICEOSCOPY 

brilliancy,  to  succeed  in  a  scientific  study ;  work  at  it  ardently 
and  perseveringly,  and  success  will  follow.' 

In  order  that  the  best  results  may  be  obtained,  there  must  be 
a  correct  understanding  of  the  general  principles  on  which  the 
instrument  should  be  worked,  and  the  equipment  so  selected 
that  each  part  is  in  tune  with  the  other,  and  by  co-operative 
working  in  skilful  and  interested  hands  the  whole  may  give  the 
maximum  effect.  It  is  no  uncommon  thing  to  find  work  ill-done 
through  insufficient  appreciation  of  elementary  facts ;  it  is  used 
too  much  as  a  tool,  and  too  little  as  an  instrument.  This  applies 
particularly  to  the  professional  worker,  who  frequently  has  routine 
work  to  do,  and  does  not  concern  himself  with  half  the  possi- 
bilities of  his  microscope,  so  long  as  he  is  able  to  do  what  is 
immediately  necessary  in  the  most  rapid  manner.  The  worker 
who  is  wisely  equipped,  and  brings  to  his  study  the  intelligence 
that  is  necessary,  will  cause  his  microscope  to  reveal  to  him  the 
best  it  is  capable  of,  and  will  find  a  fresh  delight  in  each  new 
structure  that  is  unfolded  in  all  its  striking  detail  to  him. 

In  the  succeeding  pages  the  use  and  proper  place  of  both  the 
microscope  and  its  accessories  are  indicated  in  the  plainest 
manner,  and  if  the  rules  of  manipulation  which  are  given  are 
followed,  success  is  insured. 


PART    I 

CHAPTEK  I 

THE  MICROSCOPE-STAND 

As  one  looks  through  the  catalogues  of  the  various  dealers,  and 
notices  the  microscope- stands  varying  in  price  from  £2  to  £40, 
a  feeling  of  bewilderment  arises  as  to  what  is  essential  and  what 
can  be  dispensed  with.  We  will,  then,  examine  the  parts, 
describe  their  uses  and  advantages,  and  state  what  is  necessary 
for  a  beginner. 

Here  let  us  advise  intending  purchasers  not  to  buy  a  micro- 
scope unless  it  bear  the  name  of  a  well-known  manufacturer  :  a 
good  workman  is  never  ashamed  of  his  handiwork.  There  are 
many  very  inferior  instruments  that  look  tempting,  but  a 
practical  acquaintance  with  them  soon  discovers  their  weak 
points  and  inefficiency.  If  the  user  is  at  all  progressive,  an 
instrument  of  inferior  quality  is  either  speedily  discarded  in 
favour  of  a  well-made  one,  or  it  may,  on  the  other  hand,  cause 
him  to  become  disheartened,  and  attribute  want  of  success  to  his 
own  incapacity  instead  of  the  poor  quality  of  the  instrument. 

Although  a  good  second-hand  instrument  may  be  occasionally 
met  with,  great  discretion  is  required  in  purchasing,  because 
improvements  may  have  been  introduced  since  its  manufacture, 
or  some  damage  may  have  occurred  to  the  optical  parts.  If  it 
be  obtained  from  a  respectable  dealer  who  understands  his 
business,  and  will  give  a  guarantee  of  condition,  there  is  some 
inducement ;  but  a  friend  who  is  up  to  date  in  microscopy  is 
generally  the  best  to  advise.  In  all  cases  before  purchasing,  a 
catalogue  should  be  obtained  from  the  maker  whose  name  the 
instrument  bears,  so  that  it  may  be  ascertained  whether  the 


6  MODERN  MICROSCOPY 

pattern  is  still  made,  or  is  antiquated  and  out  of  date.  It  is 
much  better  to  buy  a  good  stand,  capable  and  worthy  of  receiving 
additional  apparatus  from  time  to  time,  rather  than  an  inferior 
instrument  that  is  completely  furnished  with  objectives  and 
accessories.  These  latter  rarely  engender  pride  of  ownership, 
and  are  often  relegated  to  some  obscure  corner  after  a  short 
acquaintance ;  whereas,  if  a  good  instrument  be  purchased,  with 
but  one  objective  to  start  with,  there  is  always  a  pleasure  in 
working  with  it,  and  a  peculiar  fascination  from  its  quality — a 
satisfaction  in  feeling  that  one  has  something  superior. 

A  microscope-stand  of  large  size  is  frequently  referred  to  as 
'  powerful ' ;  however,  the  magnifying  power  does  not  depend  on 
this  quality,  but  on  the  optical  parts — the  eyepiece  and  objectives 
which  are  used.  The  tendency  in  modern  instruments  has  been 
to  minimize  the  size,  for  the  reason,  no  doubt,  that  workers  in 
laboratories  use  them  in  the  upright  position,  and  inconvenience 
in  looking  down  the  tube  is  reduced,  if  the  microscope  is  as 
short  as  possible.  This  has  unfortunately  led  to  such  con- 
tracted proportions  that  efficient  working  is  considerably  inter- 
fered with.  It  is,  therefore,  important  that  the  microscope  that 
is  chosen  shall  have  sufficiency  of  height  for  all  apparatus  to  be 
freely  used  with  it. 

On  p.  7  a  typical  student's  microscope  is  figured,  by  refer- 
ence to  which  the  different  parts  of  the  instrument  will  be  made 
clear. 

Fig.  1. — A  is  the  stand,  or  foot. 

B  is  the  tailpiece  carrying  the  mirror  (C),  with  which  light  is 
thrown  upon  the  object. 

D  is  the  under-fitting,  into  which  are  fitted  the  sub-stage 
condenser,  polarizer,  etc. 

E  is  the  stage  on  which  the  object  is  placed. 

F  is  the  limb  carrying  the  body  (G). 

At  the  lower  end  of  the  body  is  a  nosepiece  (H),  having  a 
screw  into  which  the  objective  is  fitted. 

At  the  upper  end  of  the  body  is  a  sliding  fitting  called  the 
draw-tube  (J),  by  means  of  which  additional  magnification  may 
be  obtained,  and  into  this  draw-tube  the  eyepiece,  or  ocular 
(K),  fits. 

L  is  a  rackwork,  by  means  of  which  the  body  (G)  is  raised 


Fig.  1. — A  Typical  Student's  Microscope. 
The  '  Frara  '  Microscope.     By  W.  Watson  and  Sons,  Ltd. 


8  MODERN  MICROSCOPY 

and  lowered  in  order  to  focus  the  objective  upon  the  object 
which  is  placed  on  the  stage  (E). 

M  is  the  milled  head  controlling  the  fine  adjustment,  which 
imparts  a  delicate  motion  to  the  body,  in  order  that  the  objective 
may  be  more  exactly  adjusted  than  would  be  possible  with  the 
rackwork  (L)  when  using  high  magnifying  power. 

0  is  one  of  the  springs  with  which  the  object  is  held  in 
position. 

We  have  selected  the  instrument  (Fig.  1)  because,  from  prac- 
tical acquaintance  with  it,  we  are  able  to  strongly  recommend  it 
for  a  beginner's  microscope,  worthy  of  receiving  additions  from 
time  to  time  as  means  may  permit.  Still,  it  should  only  be 
considered  as  a  typical  one. 

The  Foot. 

Since  the  first  edition  of  this  book  was  issued,  a  decided 
reaction  has  taken  place  in  regard  to  the  form  of  foot  on  which 
the  microscope  is  mounted.  There  has  scarcely  been  a  modern 
writer  of  repute  who  has  not  urged  the  necessity  and  importance 
of  having  such  a  mounting  for  the  microscope  as  shall  secure 
for  it  absolute  rigidity,  whether  it  be  used  vertically,  inclined, 
or  horizontally  for  photography.  No  foot  so  fully  answers  these 
requirements  as  the  tripod  pattern.  Rarely  does  it  happen  that 
the  bench  or  table  on  which  work  is  done  is  absolutely  level, 
and  the  tripod  is  the  only  pattern  that  naturally  adjusts  itself  to 
such  inequalities  of  surface.  It  cannot,  therefore,  be  too  emphatic- 
ally insisted  that  microscopists  should  select  this  pattern  in 
preference  to  any  other.  At  first  sight  this  feature  may  appear 
to  be  a  somewhat  trivial  one,  and  especially  so  to  a  novice ;  yet 
minor  details  have  a  marked  significance  in  the  satisfactory 
execution  of  his  work.  It  will  be  found  advantageous  to  have  the 
foot  shod  with  cork,  as  thereby  the  microscope  is  in  a  degree 
insulated  from  vibration,  and  the  risk  of  scratching  the  surface 
of  the  table  on  which  it  is  being  used  is  avoided.  It  must  be 
clearly  understood,  however,  that  even  this  form  of  foot  must 
be  made  in  correct  proportion,  or  its  advantages  will  be 
minimized. 

The   Jackson  model  foot,  which  was   suited   particularly  to 


THE  MICEOSCOPE-STAND 


9 


instruments  of  large  size,  has  not  the  popularity  that  it  once 
enjoyed.     It  is,  nevertheless,  in  point  of  rigidity  and  convenient 


Fig.  2.— D.P.H.  Microscope,  by  C.  Baker,  showing  Jackson  Foum 

of  Foot. 


shape,  one  of  the  best.     It  will  be  seen  on  the  instrument  in 
Fig.  2  above. 


} 


10  MODERN  MICROSCOPY 


The  type  of  foot  known  as  the  '  horseshoe  '  is  preferred  by 
numerous  workers  all  over  the  world — we  refer  particularly  to 
medical  students  and  laboratory  workers.  To  them  it  is  quite 
satisfactory,  because,  using  the  microscope  in  the  upright  position, 
it  is  sufficiently  stable,  and  allows  convenient  access  to  the  mirror 
and  sub-stage  condenser.  It  is  also  very  compact,  and  if  uncon- 
sciously it  be  brought  over  the  front  edge  of  the  bench,  it  will 
remain  firm,  where  the  tripod  instrument  will  fall  completely 
over. 

The  amateur,  however,  generally  uses  his  microscope  inclined 
at  an  angle,  and  in  this  position  the  instrument  with  the  horse- 
shoe foot  is  invariably  top-heavy,  and  has  a  tendency  to  fall 
over  with  the  least  knock  or  pressure. 

A  modification  of  the  horseshoe  foot  has  of  recent  years  been 
introduced,  and  finds  much  favour.  It  has  all  the  advantages 
of  the  horseshoe,  and,  if  properly  proportioned,  many  of  the 
disadvantages  are  overcome.  The  proper  proportioning  is  there- 
fore of  great  importance  ;  this  is  too  frequently  overlooked. 

All  considered,  the  tripod  foot  has  so  many  points  of  advantage 
that  it  is  unhesitatingly  to  be  recommended  in  preference  to  any 
other. 

The  selection  of  the  foot  of  the  microscope  would  therefore  be 
in  the  following  order  : 

1.  The  tripod  foot,  as  fitted  to  the  instrument  illustrated 
on  p.  7. 

2.  Jackson  form  of  foot,  as  fitted  to  the  instrument  illus- 
trated on  p.  9. 

3.  The  modified  horseshoe  foot,  which  will  be  seen  on  p.  23. 

4.  The  Continental  or  horseshoe  foot,  as  shown  on  p.  17. 

The  Stage. 

The  stage  of  the  microscope  on  which  the  object  is  placed  for 
examination  may  be  divided  into  two  classes :  (1)  mechanical, 
and  (2)  plain. 

The  Mechanical  Stage. — The  instruments  figured  on  p.  25. 
and  frontispiece  are  provided  with  this  type  of  stage,  in  which, 
by  the  turning  of  two  milled  heads  which  are  attached  to  screws, 
plates  are  moved  in  dovetailed  grooves  one  over  the  other,  in 


THE  MICKOSCOPE-STAND  11 

rectangular  directions,  carrying  the  object  with  them.  A  first- 
class  microscope  should  be  provided  with  this  form  of  stage  ;  in 
fact,  there  is  no  means  so  suitable  for  systematically  examining 
an  object  as  is  afforded  by  it.  In  addition  to  these  mechanical 
movements,  if  a  bar  be  fitted  to  slide  in  a  vertical  direction  on 
the  top  plate  the  efficiency  of  the  stage  will  be  greatly  increased. 
The  mechanical  stage  lends  itself  to  the  adaptation  of  further 
important  movements.  A  means  of  rotating  the  object  is  an 
essential  in  most  classes  of  work.  For  this  purpose  the  lowest 
plate  of  the  stage  is  usually  fitted  to  rotate  on  the  fixed  centre 
of  the  base-plate,  the  mechanical  movements  acting  above  it. 
It  is  then  termed  a  concentric  rotating  stage,  the  object  remain- 
ing in  the  field  during  the  whole  rotation  of  the  stage.  In 
mechanical  stages  of  economical  construction  the  rotating  plate 
is  occasionally  fitted  above  the  mechanical  movements,  and  is 
carried  by  them,  in  which  case  it  does  not  rotate  concentrically. 
The  object  can,  notwithstanding,  be  kept  in  the  centre  of  the 
field  by  constantly  re-setting  it  with  the  mechanical  screws 
during  the  rotation  of  the  plate.  Some  stages  of  the  concentric 
form  are  arranged  to  rotate  by  rackwork  and  pinion  ;  although 
this  is  not  really  an  essential,  it  is  often  convenient ;  it  also 
prevents  the  stage  from  rotating  accidentally,  especially  while 
photographing.  When  it  is  provided,  it  should  have  the  pinion- 
wheel  so  arranged  that  it  may  be  disengaged  from  the  rack  and 
replaced  instantly. 

Centring  screws  to  the  concentric  rotating  stage,  by  means  of 
which  the  axis  of  the  stage  may  be  made  true  with  any  objective, 
will  be  found  a  useful  addition,  especially  if  petrological  work  is 
to  be  done.  Divisions  to  the  periphery  of  the  stage  for  reading 
the  angle  through  which  the  stage  is  rotated  are  not  advan- 
tageous for  ordinary  purposes,  but  for  chemical  and  petrological 
work  they  are  a  necessity. 

A  remark  is  necessary  respecting  the  free  opening  in  the 
centre  of  the  mechanical  stage.  It  is  essential  that  the  stage 
plates,  when  carried  by  the  mechanical  movements,  may  in  no 
position  be  obstructed  by  the  top  of  the  sub-stage  condenser. 
Many  of  the  sub-stage  condensers  in  use  are  of  somewhat  large 
diameter,  and,  unless  this  feature  has  received  proper  thought 
and  arrangement,  constant  difficulty  and  trouble  will  arise. 


12 


MODERN  MICROSCOPY 


Fig.  3. — Section  through  Microscope-Stand  III.,  by  Carl  Zeiss,  with 
Diagram  of  Path  of  Rays  with  Low-Power  Objectiye  in  Use,  showing 
also  Construction  of  Fine  Adjustment. 


THE  MICEOSCOPE-STAND  13 

Attachable  Mechanical  Stages. — In  recent  years  a  variety  of 
mechanical  stages,  which  can  be  attached  to  or  removed  from  an 
ordinary  plain  stage  microscope,  have  been  introduced.  Some 
of  these  possess  merit,  but,  taken  as  a  whole,  they  are  inaccurate 
in  working,  and  at  their  best  are  not  for  one  moment  to  be 
compared  with  the  mechanical  stage,  which  has  been  built  as  an 
integral  part  of  the  microscope,  and  no  microscopist  who  wishes 
to  do  himself  and  his  work  full  justice  should  use  such  a  fitting. 
They  frequently  fail  to  act  well  when  an  immersion  objective  is 
used,  especially  if  the  oil  is  thick  ;  and  are  impossible  with  an 
oil  immersion  condenser,  because  the  oil  on  the  under  surface 
of  the  object  slip  is  drawn  along  the  stage  plate,  and  sucks  up 
all  oil  by  capillary  attraction. 

For  some  reason,  Continental  manufacturers  rarely  fit  their 
instruments  with  the  mechanical  stage  as  it  is  understood  in 
England,  but  have  always  recommended  and  arranged  for  the 
adaptation  of  an  attachable  stage.  Where  mechanical  move- 
ments are  found  to  be  an  essential  for  certain  work  by  the 
possessor  of  an  instrument  with  a  plain  stage,  and  from  reasons 
of  inconvenience  or  impracticability  the  plain  stage  cannot  be 
exchanged  for  a  proper  mechanical  one,  then,  and  then  only, 
should  the  attachable  form  be  resorted  to. 

For  many  classes  of  work,  a  stage  with  a  very  long  range  of 
movement  is  found  advantageous.  Many  ingenious  devices  to 
effect  this  have  been  invented,  but  in  nearly  every  case  they 
present  drawbacks  over  stages  of  more  limited  movement,  and 
it  is  for  the  consideration  of  the  worker  whether  the  convenience 
of  the  long  range  is  of  higher  importance  to  him  than  any  slight 
disadvantage  which  he  may  have  to  suffer. 

For  this  reason  the  attachable  mechanical  stages  are  par- 
ticularly appreciated,  the  self-contained  method  of  constructing 
the  horizontal  movement  allowing  of  extra  travel  being  given 
to  the  object-carrier.  By  this  means  the  object  can  usually  be 
carried  about  2  inches. 

Several  long-range  mechanical  stages  of  the  fixed  type  are 
offered  by  opticians.  Messrs.  Swift  and  Son  and  W.  Watson 
and  Sons  both  have  excellent  designs,  the  former  in  their 
I.M.S.  microscope,  and  the  latter  in  their  Scop  and  Bactil  stages, 
all  of  which  give  2  inches  of  horizontal  movement.     Usually, 


14  MODEEN  MICROSCOPY 

however,  for  the  work  of  the  amateur,  mechanical  movements 
of  1  inch  (25  millimetres)  in  both  directions  are  found  ample. 

Finders  to  Mechanical  Stages. — Divided  scales,  reading  to 
parts  of  an  inch  or  millimetre,  fitted  to  the  plates  of  the  mechan- 
ical stage,  will  be  found  of  great  utility.  By  means  of  such  an 
arrangement,  important  parts  of  an  object  can  be  noted  and 
subsequently  refound.  For  instance,  supposing  a  specimen  were 
being  examined,  and  an  important  feature  were  observed  to 
which  future  reference  would  be  desirable,  it  would  only  be 
needful  to  take  the  reading  of  the  divisions  on  the  stage,  and 
record  them  on  the  slide — say,  horizontal,  24  ;  vertical,  20.  On 
future  occasions,  on  setting  the  stage  readings  at  the  same  points 
and  placing  the  object  in  the  same  position  on  the  stage  (for 
which  purpose  nearly  all  mechanical  stages  have  a  stop-pin, 
against  which  the  slide  can  be  set),  the  special  feature  would 
be  at  once  in  the  field  of  view.  These  divisions  can  also  be  used 
for  roughly  measuring  objects,  the  modus  operandi  of  which  is 
given  in  the  instructions  for  the  measurement  of  objects,  p.  123. 

If  a  mechanical  stage  is  selected,  it  should  be  a  good  one,  for 
if  badly  made  it  is  far  less  advantageous  than  a  plain  stage  ; 
also  the  frictional  parts  should  be  sprung,  and  fitted  with 
adjusting  screws,  so  that  compensation  may  be  made  for  wear 
and  tear. 

Plain  Stages. — The  stage  of  the  microscope  shown  on  p.  7 
has  two  flat  springs  only,  to  hold  the  object  in  position  on  the 
surface,  and  the  movement  of  the  object  is  effected  by  the  fingers. 
For  cursory  examinations  this  answers  every  purpose  ;  but  where 
systematic  work  is  to  be  done  something  more  is  needed,  and 
this,  when  a  mechanical  stage  is  not  provided,  should  take  the 
form  of  a  bar  reaching  completely  across  the  stage  and  sliding 
in  a  vertical  direction.  If  properly  fitted  and  sprung,  it  will 
travel  freely  when  gently  pressed  with  one  hand  only.  The 
object  is  carried  by  it,  and  can  be  moved  in  a  horizontal  direc- 
tion upon  this  bar.  With  a  little  practice  the  fingers  become 
educated  to  the  work,  enabling  examinations  to  be  conducted 
with  the  highest  powers  almost  as  rapidly  and  systematically  as 
with  the  mechanical  screws.  The  sliding-bar  should  further  be 
provided  with  two  flat  springs,  so  fitted  that  they  may  be  turned 
inwards   to   rest   on   the    bar  when  not  required.     It   is  often 


THE  MICROSCOPE-STAND 


15 


necessary  to  set  an  object  at  an  angle  across  the  stage  during 
observation,  in  order  that  some  special  feature  may  appear 
vertically  in  the  centre  of  the  field.  If  the  springs  are  not 
provided  this  cannot  be  done. 

Finders  for  Plain  Stages. — The  form  of  finder  suggested  by 
the  late  Mr.  Lewis  Wright  for  plain  stages  is  the  most  efficient 
for  practical  purposes.     Many  proposals  have  been  made,  but 


Fig.  4. — Attachable  Mechanical  Stage,   by  Carl  Zeiss. 

(Two-thirds  full  size.) 


none  equal  this  one  for  simplicity.  On  the  right-hand  side  of 
the  central  aperture,  one  inch  of  the  stage  is  divided  into  fifty 
parts  in  vertical  and  horizontal  directions.  A  special  feature  of 
interest  in  an  object  having  been  discovered,  the  slide  being 
maintained  in  a  horizontal  position  across  the  stage  by  means  of 
the  sliding-bar,  it  is  only  necessary  to  read  from  the  top  right- 
hand  corner  of  the  slide  the  lines  against  which  it  lies.  A  note 
of  same  is  made  on  the  labels  of  the  object,  and  the  specimen 


16  MODERN  MICROSCOPY 

can  subsequently  be  placed  in  exactly  the  same  position,  and 
the  subject  re-examined.  Without  the  sliding-bar  it  is  somewhat 
difficult  to  keep  the  object  exactly  straight  across  the  stage,  but 
with  care,  on  observing  an  important  feature,  the  slide  can  be 
gently  turned  until  it  is  in  a  correct  position  for  taking  readings. 

The  great  saving  of  time  that  is  afforded  by  such  a  device  as 
this  should  establish  its  claim  to  be  placed  on  every  student's 
microscope.  Several  makers  have  already  adopted  the  arrange- 
ment, and  it  would  be  a  great  gain  to  microscopists  in  general  if 
a  uniform  position  for  the  divisions  were  agreed  upon  between 
them,  so  that  a  person  noting  a  special  point  with  his  microscope 
could  send  the  specimen,  with  the  readings  marked  upon  it,  to  a 
brother  worker,  and  he,  having  the  same  kind  of  finder  on  his 
stage,  would  at  once  be  able  to  find  the  desired  spot. 

The  following  method  would  be  suitable  for  the  average  size 
of  stage  :  A  piece  of  metal  the  same  size  as  an  ordinary  glass 
slip  (3x1  inches)  should  be  adopted  as  a  tool,  and  f  inch  from 
one  end  and  ~  inch  from  the  edge  a  minute  spot  should  be 
made  with  a  small  drill.  The  metal  slide  should  be  placed  on 
the  stage  with  the  spot  towards  the  front,  and  the  f -inch  space 
to  the  right  of  the  centre  of  the  stage.  The  drilled  spot  should 
then  be  placed  central  in  the  field  of  a  1-inch  objective,  and  the 
outer  margin  of  the  square  of  divisions  marked  off  from  the 
right-hand  end  of  the  metal  slide. 

The  Wright's  finder  is  obviously  unsuitable  for  any  other  than 
a  stage  whose  upper  surface  does  not  travel. 

In  selecting  a  stage  for  a  microscope  our  choice  would  therefore 
be  as  follows  : 

For  a  first-class  microscope  :  Mechanical  movements ;  con- 
centric rotation ;  screws  to  make  the  rotation  quite  true  with 
any  objective  ;  sliding-bar  to  top  plate  and  stop-pin  for  object 
to  go  against ;  divisions  to  plates  of  stage  reading  to  parts  of 
millimetre  or  inch  ;  rackwork  rotation  to  stage  ;  and  (optional) 
divisions  to  periphery  of  stage. 

For  a  second-class  microscope  :  Mechanical  movements ; 
sliding-bar  to  top  plate  ;  non-concentric  rotation. 

For  a  third-class  microscope  :  Plain  stage,  with  springs  to 
hold  object  in  position  ;  if  provided  with  sliding-bar  or  plate  as 
object-carrier,  so  much  the  better. 


THE  MICEOSCOPE-STAND 


17 


The  Sub-Stage  or  Fitting  under  the  Stage  to  carry 

Condenser,  etc. 

The  Sub-Stage. — This  consists  of  a  tube  which  should  be 
1*527  inches  =  38'786  millimetres,  or,  roughly,  1£  inches  full, 
internal  diameter  —  termed  the  '  Society's  size.'  It  carries 
illuminating  apparatus  for  condensing  the  light  on  the  object, 


Fig.  5. — Bausch  and  Lome  Research  Microscope  Stand  BB,  on  Horseshoe 

Pattern  Foot. 

the  polarizing  prism,  and  other  apparatus,  referred  to  on  a  later 
page.  It  is  adjusted  in  a  vertical  direction  to  and  from  the 
under  surface  of  the  stage  by  means  of  a  rack  and  pinion,  and 
the  ring  carrying  the  apparatus  is  mounted  in  an  outer  collar 
provided  with  screws,  by  means  of  which  the  condenser,  or  other 

2 


18  MODEM  MICROSCOPY 

apparatus,  can  be  made  exactly  central  with  the  objective  with 
which  it  is  working.  This  central  fitting  is  made  to  rotate  by 
rack  and  pinion  in  some  instances  for  using  the  polarizer,  etc., 
but  this  is  so  rarely  needed  that  it  is  unnecessary  except  for 
special  work.  It  is  essential  that  the  sub-stage  should  be  sub- 
stantially made,  as  it  is  a  most  important  fitting,  often  too  little 
appreciated.  A  fine  adjustment,  to  permit  of  the  condenser 
being  focussed  in  the  most  exact  manner,  is  often  provided  with 
the  best  stands,  and  it  is  exceedingly  convenient  and  of  very 
great  importance  where  high-power  work  is  intended  to  be  done. 
Often  it  is  wished  just  to  alter  the  focus  of  the  sub-stage  con- 
denser very  slightly.  In  attempting  to  do  so  the  tension  on  the 
milled  head  of  the  rackwork  is  apt  to  cause  vibration,  so  that 
the  best  point  of  adjustment  cannot  be  at  once  observed.  By 
communicating  this  small  amount  of  motion  with  the  fine  adjust- 
ment the  focus  is  obtained  to  a  nicety.  It  is  also  especially 
convenient  where  a  number  of  specimens  have  to  be  examined. 
The  varied  thicknesses  of  the  slips  necessitate  a  slight  readjust- 
ment of  the  condenser  in  each  instance,  and  this  can  be  very 
quickly  done  if  a  fine  adjustment  be  fitted  to  the  sub-stage. 
Further,  the  modern  sub-stage  condensers  possess  such  large 
apertures  that  their  exact  adjustment  becomes  equal  in  impor- 
tance to  the  precise  focussing  of  the  objective. 

Where  a  microscope  is  provided  with  a  sub-stage  it  is  necessary 
to  ascertain  if  it  will  centre  with  the  objective  by  means  of  its 
screws ;  this  should  be  done  in  the  same  manner  as  described 
below  for  the  '  under-fitting,'  and  the  centring  screws  turned. 
Also,  it  is  very  important  that  when  the  sub-stage  is  racked  up 
or  down  it  should  maintain  its  centre  with  the  optical  axis.  But 
few  instruments  will  stand  this  test ;  in  consequence  of  untrue 
mounting  or  building  the  sub-stage  goes  out  of  centre — slightly 
in  some  cases,  considerably  in  others.  There  ought  to  be  absolute 
truth  if  everything  is  square,  and  any  great  deviation  in  this 
respect  should  call  for  rectification.  If  a  fine  adjustment  be 
fitted  to  the  sub-stage,  it  may  be  tested  by  using  the  upper 
surface  as  a  stage  and  placing  the  object  on  it ;  this  may  be 
made  to  adhere  with  a  little  tallow  or  grease.  An  objective  of 
medium  power  will  probably  not  focus,  the  sub-stage  being  too 
far  away.     The  nosepiece  end  of  the  microscope  must  therefore 


THE  MICEOSCOPE-STAND  19 

be  lengthened  ;  for  this  purpose  remove  the  prism  from  an 
analyzer  fitting,  and  use  this  fitting  as  a  lengthening  adapter. 
The  object  is  then  viewed  in  the  usual  way. 

In  the  construction  of  the  sub-stage  once  again  the  Continental 
microscopes  are  not  for  a  moment  to  be  compared  with  their 
English  contemporaries.  It  is  exceptional  for  a  maker  of  micro- 
scopes on  the  Continent  to  provide  centring  screws  to  his  sub- 
stage.  It  is  simply  impossible  to  do  good  work  without  this 
convenience.  There  is  hardly  a  worker  nowadays  who  does  not 
have  objectives  by  more  than  one  maker,  and  it  will  be  found 
that  these  have  different  centres.  Continental  opticians  maintain 
that  their  method  simplifies  work,  for  the  condenser  is  centred 
once  for  all  to  the  objectives  that  are  supplied  with  the  instru- 
ments. Simple  and  obvious  tests  will  quickly  demonstrate  that 
this  is  quite  fallacious. 

The  Under-Fitting. — In  the  cheaper  instruments,  instead  of 
the  sub-stage  as  above  described,  an  ordinary  plain  tube,  termed 
the  under-fitting,  is  screwed  into  the  under  side  of  the  stage,  and 
in  this  tube  the  condenser  or  other  apparatus  is  moved  up  and 
down  to  focus.  It  is  shown  fitted  to  the  microscope  figured  on 
p.  7.  This  must  be  truly  centred  with  the  optical  tube,  and 
it  is  well  to  test  it  by  placing  a  small  diaphragm  in  the  under- 
fitting,  and  with  an  objective  in  the  body  to  focus  the  diaphragm. 
If  it  is  not  central  it  is  practically  useless.  The  additional  con- 
venience and  necessity  of  the  centring  sub-stage  cannot  be  too 
fully  impressed  upon  the  beginner  who  contemplates  doing 
thorough  work. 

Mention  must  be  made  of  a  useful  addition  to  the  under-fitting 
tube,  which  consists  of  a  simple  but  effective  means  of  raising  and 
lowering  it  so  that  the  condenser  may  be  focussed.  The  under- 
fitting  is  mounted  on  an  arm  which  is  strongly  attached  to  a 
substantial  screw  fixed  to  the  under  side  of  the  stage,  the  turning 
of  which  effects  the  purpose.  It  will  be  seen  on  the  microscope 
figured  on  p.  17.  Its  efficiency  and  that  of  all  under-fittings 
would  be  greatly  increased  if  centring  screws  were  provided. 

The  great  convenience  will  be  found  in  many  instruments  of 
being  able  to  swing  the  sub-stage  aside  out  of  the  optical  axis 
of  the  instrument  on  a  hinge-joint  fitting.  It  saves  much  time 
to  students,  especially  where  two  or  three  powers  are  constantly 


20  MODERN  MICROSCOPY 

being  interchanged,  and  the  condenser  may  not  be  required  for 
all  of  them.  Where  this  arrangement  exists  it  should  be  adapted 
in  a  workman-like  and  substantial  manner,  and  a  proper  support 
given  to  the  fitting  when  in  the  optical  axis  to  make  it  perfectly 
rigid. 

The  choice  with  regard  to  a  sub-stage  would  therefore  be — 

In  a  first-class  microscope :  Sub-stage,  having  rackwork  and 
fine  adjustment  for  focussing,  and  provided  with  facilities  for 
centring ;  rackwork  rotation,  if  for  examination  of  crystals  or 
for  petrology. 

Second-class  instrument :  Sub-stage,  having  rackwork  and 
centring  adjustments,  and  means  of  lifting  aside  out  of  the 
optical  axis. 

Student's  instrument :  The  same  as  the  second-class,  or  with 
the  plain  under-fitting,  with  or  without  screw  focussing  arrange- 
ment ;  preferably  with  centring  arrangement.  In  any  case  it 
is  imperative  that  it  shall  be  of  the  '  Universal '  size. 

Arrangements  for  Focussing. 

Although  other  parts  have  their  relative  value  in  producing 
efficient  working,  the  most  important  are  those  which  provide 
the  means  for  focussing  the  object-glass.  Those  who  have  been 
troubled  with  movements  which  have  exhausted  patience  and 
prevented  the  execution  of  work  in  hand,  will  sympathize  with 
the  insistence  on  sound  principles  of  construction  and  perfect 
workmanship. 

Two  adjustments  are  provided  for  focussing — one  called  the 
coarse  adjustment,  and  the  other  the  fine  adjustment.  The 
former  is  usually  so  well  made  that  it  is  possible  to  focus  high- 
power  objectives  precisely  with  it.  It  is  by  itself  sufficient  for 
low-power  work.  The  use  of  the  fine  adjustment  is  for  bringing 
to  the  sharpest  possible  focus  the  details  of  the  objects  which  are 
being  examined  with  high-power  objectives,  and  when  it  is  men- 
tioned that  the  amount  of  movement  required  to  be  imparted 
is  frequently  only  TqVo  °f  an  inch,  the  exactness  necessary  will 
be  in  some  degree  appreciated. 

The  Coarse  Adjustment. — There  is  only  one  type  of  coarse 
adjustment  now  fitted  to  all  instruments,  and  this  consists  of  a 


THE  MICROSCOPE-STAND 


21 


rack  and  pinion  actuating  the  body  in  a  very  true-fitting  dove- 
tailed bearing,  as  per  Fig.  6  shown  on  p.  21.  In  the  illustration 
it  will  be  seen  that  the  rack  is  cut  diagonally,  and  the  pinion 
corresponds.  In  order  that  it  may  work  at  its  best,  each  tooth 
of  the  rack  has  to  be  carefully  '  ground  in  ' — that  is,  fitted  to  a 
leaf  in  the  pinion — and  so  that  the  fitted  tooth  of  the  rack  may 
always  engage  the  correct  leaf  of  the  pinion,  it  is  necessary  so 


Fig.  6.  —  Showing  the  Arrangement 
of  Rack  and  Pinion,  and  Fine 
Adjustment  Dovetailed  Fitting 
with  Adjusting  Screws  A. 


Fig.  7. — Sectional  View  of  Ad" 
justable  Fittings  of  Rack  and 
Pinion. 


to  fix  the  body  that,  when  racked  up  as  high  as  possible,  it 
may  not  be  withdrawn  from  its  bearings  and  rackwork,  it  being 
usually  provided  with  a  '  stop  '  screw.  The  pinion  should  have 
suitable  provision  by  means  of  adjusting  screws  for  exactly  con- 
trolling the  stiffness  of  the  rackwork  action,  and  for  taking  up 
slight  backlash  which  may  arise  in  consequence  of  wear  and  tear. 


22  MODERN  MICROSCOPY 

An  illustration  of  the  method  adopted  to  secure  this  result  is 
shown  in  Fig.  7,  p.  21,  in  which  the  pinion  P  is  held  in  position 
by  a  block  of  metal,  N,  against  which  pressure  is  exerted  by 
two  screws,  one  of  which,  M,  is  shown  in  the  figure.  With  this 
arrangement  the  most  exact  relation  of  pinion  to  rack  can  be 
established  and  maintained. 

For  the  purposes  of  the  amateur  a  long  range  of  coarse 
adjustment  is  necessary,  so  that  low-power  objectives  can  be 
used.  It  should  allow  of  a  distance  of  3  to  4  inches  between  the 
nosepiece  and  the  stage  when  the  body  is  racked  to  its  highest 
point.  The  shortness  of  the  movement  in  the  instruments  of 
Continental  manufacture  renders  these  instruments  generally 
unsuitable  for  the  all-round  work  of  the  ordinary  microscopist. 

It  has  been  recommended  that  microscopists  should  take  their 
instruments  to  pieces  in  order  that  they  may  judge  of  their 
workmanship  ;  but  in  reality  a  well-made  microscope  requires 
to  be  as  carefully  put  together  as  a  watch,  and  for  a  novice  to 
attempt  to  undo  the  parts  means  very  probable  detriment  to  the 
instrument.  The  name  of  a  first-class  maker  on  an  instrument 
may  generally  be  considered  a  guarantee  of  good  workmanship, 
otherwise  he  could  not  possibly  maintain  his  reputation. 

Formerly  cheap  students'  microscopes,  instead  of  being  pro- 
vided with  a  rack  and  pinion  for  the  coarse  adjustment  of  the 
object-glass,  were  made  with  the  body  to  slide  in  a  fixed  tube. 
This  is  a  very  rough-and-ready  arrangement,  and  accuracy  of 
centring  cannot  be  maintained  as  with  a  rack  and  pinion. 

Fine  Adjustments. 

Well-defined  attempts  have  been  made  by  nearly  all  makers 
to  improve  this  most  important  of  all  movements.  The  demand 
for  accuracy  in  this  particular  has  been  greatly  increased  by  the 
growing  use  of  objectives  of  large  aperture  which  cannot  be 
profitably  employed  excepting  with  a  fine  adjustment  of  the 
utmost  precision.  Its  essentials  are  that  it  impart  a  very  slow 
motion  and  be  absolutely  free  from  lateral  movement.  The  fine 
adjustment  that  for  many  years  has  proved  thoroughly  satis- 
factory in  the  writer's  hands  is  that  made  by  Messrs.  Watson 
and    Sons,  also  supplied   by  other  makers,   in  some   instances 


THE  MICROSCOPE-STAND 


23 


Fig.  8. — Stand  F.  by  Leitz,  with  Modified  Horseshoe  Foot. 


with  modifications.     It  is  shown  in  position  on  the  instrument 
(Fig.  1)  and  the  working  details  will  he  gleaned  from  Fig.  9. 
The  body  is  raised  or  lowered  in  a  dovetailed  fitting  by  means 


24 


MODEEN  MICROSCOPY 


of  a  lever  contained  within  the  limb  of  the  instrument,  and  a 
pin  passing  through  it  transversely  acts  as  a  fulcrum.  By 
turning  a  milled  head  attached  to  a  micrometer  screw,  force 
is  applied  to  the  lever  at  one  end  against  a  pointed  rod,  attached 
to  the  body  and  entwined  by  a  coil  spring,  at  the  other  extremity. 
As  the  body  moves  upwards,  the  spring  is  compressed  against  a 
brass  plate,  and  on  the  micrometer  screw  being  released  this 

spring  produces  the  reactionary  power. 
One  arm  of  the  lever  is  four  and  a 
half  times  longer  than  the  other,  con- 
sequently the  weight  of  the  body  at 
the  milled-head  end  of  the  lever  and 
the  motion  imparted  are  reduced  in 
this  ratio.  Thus  the  makers  give  the 
weight  of  a  body  of  one  of  their  in- 
struments as  17  ounces,  and  this 
divided  by  4|  reduces  the  resistance 
to  3t>  ounces.  It  will  be  seen  from 
this  that  when  in  operation,  if  per- 
chance the  objective  touch  the  thin 
cover-glass  of  the  object,  it  will  do  so 
with  a  pressure  so  gentle  and  slight 
that  there  is  little  risk  of  damage  to 
either.  This  system  has  the  advan- 
tage that  the  position  of  the  milled 
head  on  the  limb  is  convenient  for 
manipulation,  and  is  not  altered 
when  the  body  is  racked  up — that  is, 
it  is  not  carried  by  the  rackwork,  as 
in  many  forms,  so  that  its  attachment  to  a  focussing  rod  of  a 
camera  for  photo-micrography  is  easy  and  convenient.  There  is 
also  a  very  simple  means  of  adjustment  provided  for  taking  up 
any  slackness  through  wear.  The  slide  in  which  the  fine  adjust- 
ment is  fitted  has  slots,  to  which  are  fitted  screws  (shown  in 
Fig.  6,  p.  21,  marked  A).  By  turning  these  screws  slightly, 
the  spring-fitting  grips  the  bearing-part  more  tightly,  and  so 
takes  up  any  wear  caused  by  friction.  Any  microscopist  can 
thereby  adjust  his  own  instrument. 
j»y  M  f  or^simnlicity,  freedom  from  complexity,  directness  of  action, 

If.  c.  Stak  c&m 


Fig.  9.  —  Section  of  Limb, 
showing  Construction  of 
Lever  Fine  Adjustment, 
etc. 

C,  lever  ;  D,  fulcrum  of  lever 
of  fine  adjustment. 


THE  MICROSCOPE-STAND 


25 


Fig.  10.— '  Imperial'  Microscope,  by  R.  and  J.  Beck,  showing  Two-Speed 

Fixe  Adjustment. 


26  MODERN  MICROSCOPY 

and  complete  effectiveness  and  convenience,  it  is  unsurpassed. 
Makers  of  stands  in  which  a  direct  acting  screw  is  fitted,  as  in 
the  Continental  form,  have  exercised  considerable  ingenuity  in 
providing  an  efficient  movement.  Carl  Reichert,  of  Vienna,  who 
has  in  recent  years  shown  highly  progressive  tendencies,  designed 
a  lever  form  of  fine  adjustment  for  this  latter  description  of 
microscope,  which  has  proved  even  more  satisfactory  than  was 
originally  anticipated.  Messrs.  Swift  and  Son,  too,  have  adopted 
a  somewhat  similar  plan  with  corresponding  success  in  their 
'  Ariston '  pattern,  while  Messrs.  Zeiss  and  Leitz  have  devised 
entirely  distinct  fine  adjustments  for  their  stands,  actuated  by 
means  of  gear-wheels,  which  produce  an  extremely  slow  and 
precise  movement. 

Powell  and  Lealand's  instruments  are  also  provided  with  a 
fine  adjustment  having  special  merit,  consisting  of  a  lever 
actuating  a  long  tube  sliding  up  and  down  inside  the  body. 

Two-Speed  Fine  Adjustments.— Complaint  is  made  that  the 
tendency  to  fit  very  slow-acting  fine  adjustments  has  become  a 
source  of  inconvenience  to  students  and  others  who  require  to 
work  rapidly  with  objectives  of  different  powers  fitted  on  revolv- 
ing nosepieces.  To  meet  this  Mr.  A.  Ashe,  of  the  Quekett  Club, 
designed  a  two-speed  fine  adjustment,  and  the  plan  was  carried 
to  a  practical  issue  by  Messrs.  R.  and  J.  Beck,  who  now  apply  it 
to  certain  of  their  instruments.  It  is  shown  fitted  to  the  micro- 
scope (illustrated  p.  25).  It  will  be  noticed  that  two  milled 
heads  are  provided  for  fine  focussing  instead  of  the  usual  one. 
The  upper  milled  head  turns  a  screw  having  a  coarse  thread, 
moving  the  body  ^V  inch  for  each  revolution  ;  the  lower  actuates 
a  fine  screw  which  causes  a  movement  of  the  body  oj^  inch 
for  each  complete  turn.  At  any  moment  either  milled  head 
may  be  used  and  time  thereby  saved  when  low  powers  are 
employed. 

Where  a  simpler  method  may  be  desired,  practically  the  same 
result  can  be  secured  by  having  attached  to  the  centre  of  the 
ordinary  milled  head  a  smaller  spindle,  say,  one-sixth  of  the 
diameter  of  the  former.  When  a  quick  movement  is  required 
this  spindle  can  be  turned  between  the  fingers,  and  a  rate  equal 
to  six  to  one  of  that  obtainable  by  slowly  rotating  the  ordinary 
milled  head  will  be  secured.     Messrs.  Watson  and   Sons  have 


THE  MICROSCOPE-STAND  27 

applied  this  to  their  instruments,  and  the  writer  has  found  the 
working  exceedingly  satisfactory.     It  is  shown  in  Fig.  11. 

When  testing  the  performance  of  the  fine  adjustment,  a  central 
cone  of  light  must  be  used  ;  if  the  light  be  thrown  obliquely 
there  will  be  of  necessity  an  apparent  move- 
ment in  the   direction  from  which  the  light 
comes.      With    central     illumination     there 
should  be   no  shake   or  displacement   what-         £^~ -JpHS^ 
ever  in  the  object  when  it  is  focussed.  lj 

Nearly  every  maker  has   his  own    system  JJ 

or    systems    of    fine    adjustment,    possessing  |5- 

features  more  or  less  desirable,  but  they  are  ^ 

mostly  modifications  of  those  mentioned  here.      Fl?;  llm  —  SpiNDLE 
J  Milled  Head  foe 

Some  firms  adopt  a  most  excellent  form  of  giving  Two 
fine  adjustment  for  a  superior  class  of  micro-  amu^tment. 
scope ;  while  in  the  students'  patterns  the 
method  employed  is  dissimilar  and  oftentimes  useless  for  high- 
class  work.  It  would  be  far  better  that  efficiency  were  not 
sacrificed  in  such  a  manner  for  the  small  saving  in  cost  involved. 
Above  all  things  avoid  the  form  of  fine  adjustment  which 
carries  the  whole  weight  of  the  body  of  the  instrument,  or 
depresses  it  against  a  spring,  as  in  some  Continental  and  cheap 
students'  instruments :  these  are  almost  worse  than  no  fine 
adjustment  at  all,  as  they  invariably  soon  work  loose  in  the 
fittings  and  cause  great  annoyance. 

The  Limb. 

The  design  of  the  limb  of  the  microscope  is  of  special  impor- 
tance, because  it  carries  the  body  and  is  intimately  associated 
with  the  fine  adjustment.  It  should  be  of  substantial  shape 
and  strongly  made.  In  our  opinion  the  pattern  which  is  shown 
in  the  build  of  microscopes  on  pp.  9,  25,  etc.,  is  to  be  preferred 
before  that  shown  on  p.  17,  because  in  the  former  additional 
solidity  is  imparted  to  the  body  fittings  on  account  of  there  being 
no  separate  adjustment  which  has  to  act  and  re-act  at  the  back 
part — in  other  words,  a  limb  which  carries  the  body  at  one 
extremity,  and  at  the  other  is  acted  upon  by  the  fine  adjustment 
through  a  pillar,  cannot  in  the  nature  of  things  be  so  satisfactory 


28  MODERN  MICROSCOPY 

as  a  limb  which  carries  the  fine  adjustment  instead  of  being 
supported  by  it.  Still,  it  cannot  be  denied  that  the  method  of 
attaching  the  limb  to  the  pillar  adopted  by  the  majority  of  the 
Continental  makers,  and,  for  the  matter  of  that,  those  English 
manufacturers  who  include  this  style  of  instrument  amongst 
their  models,  is  usually  a  very  substantial  one. 

In  many  recent  models  a  handle  has  been  embodied  in  the 
limb  for  carrying  the  microscope — sometimes  it  forms  a  part 
of  the  foot  pillar.  This  originated  in  consequence  of  disabilities 
associated  with  the  direct  acting  pillar  fine  adjustment.  With 
this  the  limb  is  drawn  up  the  pillar  on  the  fine  adjustment 
fitting,  when  the  instrument  is  lifted  by  the  limb  ;  and  when  set 
down  it  sometimes  happened  that  the  fingers  would  be  caught 
between  the  back  of  the  stage  and  the  limb,  for  immediately  the 
part  was  released  it  would  go  back  to  its  proper  position  by  the 
pressure  on  the  reacting  spring.  Incidentally  a  strain  was  put  on 
the  fine  adjustment  slide  in  the  lifting  process.  Generally  these 
handles  are  not  of  real  value,  because  they  are  not  sufficiently 
large  for  more  than  two  or  three  fingers,  and  the  balance  of  the 
instrument  is  such  that  it  cannot  well  be  carried  upright  by  its 
means.  Such  a  contrivance  is  not  required  where  a  fine  adjust- 
ment of  lever  pattern,  or  actuated  by  gear-wheels,  is  fitted, 
because  none  of  the  mechanism  is  interfered  with  when  the 
limb  is  used,  and  appropriately  so,  for  lifting  the  instrument. 

The   Body-Tube. 

In  the  majority  of  microscopes  of  British  make,  the  diameter 
of  the  body-tube  is  considerably  larger  than  is  exactly  necessary 
for  the  draw-tube  which  carries  the  eyepiece.  In  the  large 
microscope-stands  of  a  few  years  ago  the  body-tube  had  of 
necessity  to  be  large,  because  the  eyepieces  were  generally  of 
what  is  known  as  the  capped  pattern,  and  corresponded  in  size 
with  the  build  of  the  instrument,  and  the  large- sized  eyepieces 
are  still  used  in  expensive  instruments.  The  student's,  or  plain 
type  of  eyepiece  of  small  diameter,  is,  however,  far  more  popular, 
and  this  requires  only  a  small  fitting.  The  Continental  makers 
usually  provide  a  body- tube  only  so  much  larger  than  the  draw- 
tube  as  is  necessary  to  accommodate  it ;  but  the  large  body-tube 


THE  MICROSCOPE-STAND 


20 


is  of  great  convenience  for  photographic  work,  particularly  when 
no  eyepiece  is  used.  It  also  permits  of  the  unrestricted  effect  of 
the  special  photographic  lenses  that  are  sometimes  used  with  a 
microscope,  to  be  obtained,  because  when  the  draw-tube  is 
removed   for   such   purposes    there   is   no    '  cutting-off '    of   the 


Eys  Lens 


Diaphrag 


Field  lens.. 


Body  Tube^[ 
Draw  Tube 


Standard  Objective. 
thread 


T/T"  oil  Immersion 
Objective 


Oil  Film 
Cover  Glas 


SliW 


e»  Piece 


Draw  Tube 
diaphragm,  with 
tandard  Objective 
throad 


Objective 
Mount 


Fig.  12. — Sectional  View  of  Body  of  Microscope,  showing  Construction  of 
Various  Parts  and  Method  of  Measuring  Body-Length  approximately. 


marginal  rays  by  the  body.  Further,  no  worker  knows  what 
he  may  ultimately  wish  to  do ;  but,  having  a  large  diameter  of 
body- tube,  he  can  at  any  time  provide  himself  with  a  draw-tube 
which  will  carry  eyepieces  of  larger  diameter.  The  large  outer 
body-tube  is  therefore  recommended. 

The  length  of  the  body  has  next  to  be  considered.     There  is  a 


30  MODERN  MICROSCOPY 

growing  tendency  to  diminish  the  total  length  of  the  body,  and 
with  it  the  extension  that  is  possible  with  the  draw-tube,  for 
obviously  a  short  body  entails  a  short  draw-tube.  The  length 
of  the  tube  should  be  such  that  when  the  draw-tube  is  fully 
extended,  the  total  over-all  length  shall  be  250  millimetres. 
This  permits  of  the  effective  use  of  a  very  large  range  of 
objectives,  and  gives  latitude  for  adjustment  for  thickness  of 
cover-glass.     This  matter  is  referred  to  fully  on  p.  69. 

The  accompanying  diagram  gives  a  sectional  view  of  a  body 
of  a  microscope,  showing  a  sufficiently  accurate  method  of 
measuring  the  body-length.* 

In  some  expensive  microscopes  of  the  best  type,  in  order  to 
give  the  fullest  latitude  possible,  the  body  is  made  with  an 
over-all  measurement  of  about  140  millimetres  (5 \  inches).  It 
then  carries  two  draw-tubes,  by  the  extension  of  which,  as  much 
as  310  millimetres  (12  inches)  can  be  obtained.  One  of  the 
two  draw-tubes  is  moved  by  rackwork  and  pinion,  and  permits 
of  great  exactness  of  adjustment,  both  for  tube-length  and  for 
cover-glass  thickness. 

This  form  of  body  is  coming  more  and  more  into  use,  and 
will  be  found  a  very  great  convenience  to  the  all-round  worker. 
No  precise  advice  can  be  given  without  knowing  the  work 
intended  to  be  done,  but,  generally  speaking,  the  short  body 
with  the  two  draw-tubes  is  much  to  be  preferred  to  any  other. 

The  draw-tube  usually  has  a  scale  of  divisions  engraved  upon 
it  to  parts  of  a  centimetre  or  inch.  The  object  of  these  divisions 
is  to  enable  a  record  to  be  kept  of  magnifications  at  different 
points  of  extension,  or  a  note  to  be  made  of  the  lengths  of  tube 
that  give  the  most  perfect  corrections  for  certain  objects  and 
objectives. 

In  all  microscopes  of  medium  or  high  class,  the  universal 
screw-thread  should  be  fitted  to  the  lower  end  of  the  draw-tube ; 
where  there  are  two  draw-tubes  it  should  be  supplied  to  the 
outer  one.  The  advantages  of  this  adapter  are  numerous.  A 
low-power  objective  can  be  used  in  it  which  it  is  often  impossible 
to  focus  on  many  stands,  owing  to  the  compactness  of  the  build 
and  shortness  of  the  movement  of  the  coarse  adjustment.  With 
the  two  draw-tubes,  if  the  outer  one  have  this  adapter  fitted, 
*  For  detailed  explanation  see  footnote,  p.  54. 


THE  MICROSCOPE-STAND  31 

nearly  10  inches  of  separation  can  be  obtained  between  the 
eyepiece  and  the  objective.  It  is  further  useful  for  carrying  the 
apertometer  objective  and  the  analyzer,  described  respectively  on 
pp.  64  and  108 ;  also  the  Bertrand's  lens  for  examining  the 
'  brushes '  of  crystals,  and  for  many  other  purposes. 

It  has  occurred  within  the  experience  of  the  writer  that  results 
obtained  on  a  microscope  having  a  large  tube  could  not  be  repro- 
duced with  the  same  objective  on  an  instrument  having  a  small 
tube.  This  was  traced  to  be  due  to  the  diaphragm  at  the  bottom 
of  the  draw-tube,  and  it  has  since  been  found  that  in  many 
students'  stands  the  opening  of  this  diaphragm  is  as  small  as 
f  inch.  This  is  altogether  insufficient,  and  causes  restriction 
to  the  passage  of  rays  from  the  objective.  It  will  be  well  to 
see  that  this  diaphragm  has  an  opening  of  at  least  J  inch. 

Tailpiece  and  Mirrors. 

The  Tailpiece. — It  will  usually  be  found  convenient  if  the 
arm,  or  tailpiece,  which  carries  the  mirrors,  be  so  mounted  as 
to  be  turned  aside  with  the  mirrors  when  desired.  This  arrange- 
ment is  of  great  utility,  for  it  permits  of  light  being  readily 
directed  from  the  lamp  through  the  sub-stage  condenser  for 
critical  work.  Opticians  favour  a  rectangular  rather  than  cylin- 
drical tailpiece  to  carry  the  mirror  gimbal ;  the  reason  for  this 
is  a  little  doubtful,  but  there  is  probably  no  distinct  advantage 
in  one  over  the  other.  Where,  however,  a  cylindrical  tailpiece 
is  provided,  it  will  be  obvious  that  the  mirror  could  quickly  be 
swung  round  out  of  the  axis  of  the  microscope  and  so  obviate 
the  necessity  for  the  swinging  of  the  tailpiece  itself,  but  this  is 
quite  a  minor  consideration. 

The  Mirrors  should  be  plane  and  concave,  hung  in  a  gimbal, 
giving  universal  movements,  and  have  a  means  of  adjustment  to 
focus  in  a  vertical  direction.  The  plane  mirror  is  always  used 
with  the  condenser,  spot  lens,  etc.,  and  with  very  low-power 
objectives,  but  the  concave,  when  the  condenser  is  not  employed 
and  the  maximum  amount  of  light  is  desired. 

A  little  experiment  will  show  why  this  is  so.  Set  up  a  micro- 
scope with  a  1-inch  objective  of  moderate  aperture,  transmit 
light  from  a  plane  mirror,  then  remove  the  eyepiece,  look  down 


32  MODEKN  MICROSCOPY 

the  body- tube,  and  observe  the  extent  to  which  the  back  lens  of 
the  objective  is  flooded  with  light.  Repeat  the  experiment  with 
a  concave  mirror.  By  this  means  it  will  be  seen  that,  whereas 
with  the  plane  mirror  the  lens  is  incompletely  illuminated,  with 
the  concave  it  is  rilled  with  light.  It  is  important  that  the 
illuminant  shall  be  in  such  a  position  relatively  to  the  concave 
mirror  that  when  the  latter  is  in  use  it  will  be  as  nearly  as 
possible  at  right  angles  with  the  optical  axis  of  the  instrument. 

A  source  of  trouble  and  annoyance  in  accurate  work  frequently 
arises  from  the  imperfect  surface  of  the  plane  mirror.  It  some- 
times happens  that  it  produces  several  reflections  of  the  edge  of 
the  lamp-flame.  This  can  be  to  a  large  extent  overcome  by  the 
use  of  a  rather  more  costly  parallel-worked  mirror,  but  with  this 
there  must  be  two  reflections — the  faint  image  from  the  upper 
surface,  and  the  bright  one  from  the  lower  silvered  surface.  In 
consequence  of  this,  critical  workers  eliminate  the  mirror,  and 
set  the  microscope  so  that  the  light  from  the  illuminant  passes 
directly  into  the  condenser. 

The  parallelism  of  a  mirror  may  be  tested  by  holding  it  just 
below  the  level  of  the  eye  in  the  direction  of  a  row  of  objects, 
such,  for  instance,  as  chimney-pots ;  and  on  observing  the 
reflections,  each  subject  should  stand  out  singly  and  clearly.  If 
the  mirror  is  not  parallel-worked,  several  reflections  of  the  same 
object  will  appear  superimposed  in  the  mirror. 

Care  is  needful  in  the  use  of  the  concave  mirror,  if  the  best 
result  is  to  be  obtained  with  it.  It  should  be  so  arranged  that 
the  apex  of  the  cone  of  rays  that  it  transmits  may  be  exactly  in 
focus  on  the  object.  Many  microscopes  are  provided  with  mirrors 
that  are  unsuited  to  the  instrument,  being  either  too  long  or  too 
short  in  focus,  and  consequently  do  not  produce  good  effects.  To 
test  the  mirror,  a  piece  of  white  paper  should  be  placed  upon 
the  stage  of  the  instrument,  which  must  be  set  horizontally,  and 
light  from  a  lamp  reflected  by  the  concave  mirror  on  this  ;  then, 
by  sliding  the  mirror  up  and  down  on  the  tailpiece,  it  can  quickly 
be  seen  if  the  focal  point  can  be  obtained  upon  the  paper. 


THE  MICKOSCOPE-STAND  33 

Binocular  Microscopes. 

We  have  hitherto  been  treating  principally  of  the  monocular 
microscope,  and  this,  it  must  be  understood,  is  the  only  form 
that  can  be  used  for  critical  high-power  work.     Continental  firms 
as  a  rule  do  not  make  binocular  microscopes  of  the  usual  kind, 
but  two  or  three  of  them  make  a  special  pattern  for  dissecting, 
and  a  binocular  eyepiece,  which  will  be  found  described  under 
the  head  of  eyepieces.     The  advantage  of  a  binocular  microscope 
is,  that  both  eyes  can  be  employed  simultaneously,  saving  the 
strain  on  the  vision  which  is  apt  to  ensue  through  the  constant 
employment  of  the  monocular  microscope,  and  the  endeavour  to 
see  in  the  best  manner  the  detail  in  the  specimens  examined.    We 
should  recommend  every  user  of  the  monocular  microscope  to 
train  himself  to  work  with  either  eye,  keeping  the  one  not  in 
use  open  ;   this  will  be  found  of  the  very  greatest  service.     The 
universally  understood   binocular  microscope   is   provided  with 
a  prism,  designed  by  Wenham,  which  admits  of  the  light  going 
up  a  direct  tube,  and  reflects  light  also  into  a  second  tube.     By 
this  means  objects  can  be  seen  more  naturally  than  with  the 
monocular  microscope,  for  the  reason  that  stereoscopic  vision 
is  obtained,  and  objects  having  a  certain  amount  of  depth  may 
be  seen  completely  with  the  binocular  microscope,  whereas  with 
the  monocular  it  would  be  necessary  to  focus  in  successive  stages 
through  the  several  planes.     Especially  is  this  true  regarding 
opaque  objects,  with  low  powers.     The  stereoscopic  binocular 
conveys  an   impression  of   the  objects  viewed   that   is   almost 
startling  in  its  beautiful  effect.     Subjects  stand  out   in  relief, 
exhibiting  their  natural  contour,  and  at  once  the  worker  is  able 
to  decide  the  shape  and   form  of   an  object   in  a  way  that  it 
is   impossible   to  do  by  focussing  each  separate  plane  with  a 
monocular  instrument.     The  binocular  microscope  is  par  excel- 
lence the  instrument  for  the  amateur.     To  him  the  beautiful 
appeals  in  a  manner  that  it  perforce  cannot  do  to  the  scientific 
man,  who,  being  intent  on  the  pursuit  of   knowledge  of   some 
obscure  point,  has  no  time   to  notice,  or  if   to  notice,  cannot 
linger  to  reflect  upon  the  aesthetic  aspect.     In  the  examination 
of   rotifers  and   other   inhabitants  of  'ponds  and   rock   pools,' 

perhaps  the  most  charming  subjects  that  the  microscope  has 

3 


34 


MODERN  MICROSCOPY 


Fig.  13. — Sectional  View  of  Body  of 
Wenham  Binocular  Microscope; 
showing  the  method  of  sliding 
the  Prism   P  out  of  the  Field  so 

THAT  THE  MONOCULAR  TUBE  ONLY   MAY 
BE   USED. 

is  rested  by  looking  into  the  blank 
not  being  illuminated  will  scarcely 
For    use    with    the    binocular 


ever  revealed,  the  microscopist 
with  a  monocular  instrument 
cannot  possibly  appreciate  and 
interpret  structure  and  move- 
ment in  the  same  accurate 
manner  that  the  binocular 
enables  him  to  do.  These 
facts  should  receive  careful 
consideration  when  a  micro- 
scope is  to  be  chosen,  but  it 
must  be  borne  in  mind  that 
the  Wenham  stereoscopic  form 
of  binocular  cannot  advan- 
tageously be  used  with  an 
objective  having  a  higher 
numerical  aperture  than  0*26. 
Dr.  Carpenter  some  time  since 
pointed  out  that  when  an 
objective  having  a  larger 
aperture  was  employed  with 
the  Wenham  binocular, 
spherical  objects  became  dis- 
torted, and  instead  of  appear- 
ing round  in  shape  they 
became  conical.  Provision 
is,  therefore,  always  made 
whereby  the  prism  may  be 
withdrawn  for  the  higher 
power  objective,  and  the  light 
then  only  goes  up  the  mono- 
cular or  straight  tube,  and  the 
instrument  is  to  all  intents 
and  purposes  as  useful  and 
convenient  as  the  monocular 
microscope,  while  the  un- 
employed eye  of  the  observer 
binocular  tube  ;  the  fact  of  its 
be  noticeable, 
microscope,    the    closer    the 


THE  MICEOSCOPE-STAND  35 

posterior   lens  of   the  objective   is   brought   to   the  prism    the 
better. 

It  must  be  understood  that  all  vision  through  the  microscope 
in  the  ordinary  way  is  inverted,  that  is,  the  object  is  seen  upside 
down.  A  very  good  form  of  binocular  microscope,  devised  by 
Stephenson  and  made  by  Swift  and  Son,  erects  the  image,  or,  in 
other  words,  enables  it  to  be  seen  the  right  way  up.  It  is  excellent 
for  dissecting  purposes,  and  high  powers  can  be  employed  with 
it ;  still,  it  cannot  be  described  as  an  all-round  microscope,  and 
would  have  to  be  classed  with  instruments  for  special  work. 
Our  advice  on  the  question  of  a  monocular  or  binocular  micro- 
scope is :  If  the  instrument  be  required  for  strictly  educational, 
scientific,  or  photographic  work,  the  monocular  must  be  chosen. 
The  bulk  of  the  general  amateur's  work  is  done  with  compara- 
tively low  powers,  and  in  such  cases  the  binocular  is  unquestion- 
ably of  advantage,  and  to  be  preferred.  If  it  is  proposed  to 
combine  scientific  with  general  work  a  good  plan  is  to  have  two 
separate  bodies — monocular  and  binocular — interchangeable  in 
the  same  bearings.  The  maximum  facility  is  then  at  the  dis- 
posal of  the  user.  It  should  be  noted  that  when  the  two  bodies 
are  chosen,  it  is  well  to  have  centring  screws  to  the  rotating 
stage,  as  described  on  p.  11,  because  the  bodies  rarely  have 
identically  the  same  centres,  and  the  stage  could  not  otherwise 
be  made  to  rotate  concentrically  with  both  bodies. 

Mention  is  made  on  a  later  page  of  the  Greenough  binocular 
microscope.  It  is  not  an  instrument  that  is  suited  for  general 
work,  being  primarily  designed  for  the  preparation  of  objects. 

Microscopes  for  Special  Purposes. 

Dissecting  Microscopes. — For  serious  work,  very  elaborate 
instruments  have  been  devised,  and  are  to  be  found  in  the 
biological  laboratories.  A  well-known  instrument  of  this  class 
made  by  Carl  Zeiss,  and  completely  fitted  with  its  auxiliary 
apparatus,  costs  over  £20.  Much  good  work  is,  however,  per- 
formed with  comparatively  inexpensive  instruments,  but  it  is 
important  that  the  rests  for  the  hands  are  sufficiently  long  to 
give  the  necessary  steadiness.  Support  must  be  given  to  the 
forearm  as  well  as  the  wrist,  and  when  the  elbows  are  on  the 


36 


MODEEN  MICROSCOPY 


table,  the  forearm  and  the  wrist  should  be  carried  upon  the  arm- 
rests of  the  instrument.  Although  this  may  appear  to  be  a 
small  matter,  it  is  really  of  great  value,  both  from  the  point  of 
view  of  comfort  and  of  good  working.  Next  it  is  desirable  that 
the  stage  shall  have  a  sufficiently  large  surface  to  accommodate 
the  dish  in  which  the  subject  for  dissection  is  placed.  The  stage 
may  have  a  surface  of  glass  or  ebonite.  The  arm  carrying  the 
magnifying  lens  should  be  freely  movable  and  preferably  double- 


Fio.  14. — A  Simple  Pattern  of  Dissecting  Microscope. 


jointed  ;  rackwork  and  pinion  to  focus  the  lens  is  also  a  necessity. 
The  light-reflector  should  be  carried  in  a  gimbal,  and  consist 
of  a  mirror  on  one  side  and  a  mat  opal  disc  on  the  other. 

A  popular  form  of  dissecting  microscope,  made  in  large  and 
small  sizes  by  many  opticians,  is  that  shown  in  the  accompanying 
illustration  (Fig.  14)  ;  and  although  it  has  several  of  the  defects 
referred  to,  experience  has  shown  that  the  general  design  is  a 
useful  one. 

With  the  aplanatic  magnifiers  that  are  usually  employed  with 
the  dissecting  microscope  the  image  is  seen  erect — that  is,  the 


THE  MICKOSCOPE-STAND  37 

right  way  up — but  the  magnifying  powers  are  necessarily  limited 
to  those  which  give  a  sufficiently  free  space  for  the  use  of  the 
dissecting  needles,  etc.  The  use  is  therefore  restricted  to  com- 
paratively low  powers.  If  the  compound  microscope  is  used — 
that  is,  an  instrument  with  an  eyepiece  as  well  as  an  objective 
— the  image  is  seen  inverted ;  but  greater  working  distance,  with 
higher  magnifying  powers,  is  secured  by  this  combination.  The 
inversion  of  the  image,  however,  rendered  its  use  impracticable. 
The  introduction  of  the  Porro  prism  erector  changed  this. 
This  consists  of  a  body  fitted  with  right-angle  prisms  in'  the 
same  manner  as  the  modern  prism  binocular  glass,  carrying  an 
eyepiece  at  one  end  and  an  objective  at  the  other,  the  prisms 
serving  to  present  the  image  the  right  way  up,  and  to  permit 
of  the  reduction  of  length  of  mounting  between  eyepiece  and 
objective.  Increased  magnifications  are  obtained  with  this 
arrangement,  with  ample  working  distance,  and  it  has  proved 
of  invaluable  assistance.  It  is  shown  on  the  excellent  type  of 
microscope  by  Zeiss  on  p.  38. 

Much  useful  work  can  be  done  by  means  of  a  simple  upright 
stand  having  a  horizontal  arm,  preferably  with  a  simple  joint, 
and  always  with  provision  for  raising  and  lowering  the  arm. 
The  magnifier  is  carried  in  a  suitable  fitting  on  the  arm,  and 
frequently  fills  all  the  requirements  of  the  amateur  worker. 

Magnifiers  for  Dissecting. — These  are  usually  made  on  a 
plan  originated  by  Steinheil,  consisting  of  three  lenses  cemented 
together,  and  the  perfection  to  which  they  have  been  brought  is 
remarkable.  There  is  no  occasion  to  indicate  special  makers  of 
these  lenses  ;  all  opticians  of  repute  supply  them  in  good  quality. 
It  is  important  to  remark  that  magnifiers  consisting  of  single 
uncorrected  lenses,  in  which  no  attempt  at  aplanatism  or  achro- 
matism is  attempted,  are  not  to  be  recommended  for  other  than 
very  elementary  work.  Only  the  centre  of  the  field  of  such  a 
lens  is  ever  useful,  while  with  the  aplanatic  form  the  subject  is 
sharp  and  clear  to  the  edge.  The  various  magnifiers  are  also 
supplied  in  mounts  for  carrying  in  the  pocket,  and  will  be  found 
useful  aids  to  the  microscopist. 

There  is  a  frequent  request  from  the  unknowing  for  magnifiers 
having  large  lenses  and  yielding  high  magnification.  Such  a 
combination   is    impossible.     It  will    be  understood   when  it  is 


38 


MODERN  MICROSCOPY 


THE  MICROSCOPE-STAND 


39 


Fig.  16.— The  Apla- 
natic  Magnifier. 


recalled  that  the  magnifying  power  of  a  lens  depends  upon  its 
focus,  and  the  focus,  again,  upon  its  radius  of  curvature.  For 
this  reason,  a  lens  of  1  inch  diameter  having  a  focus  of  1  inch 
would  be  a  hemisphere  ;  and  as  the  distortion  with  an  uncor- 
rected hemispherical  lens  would  be  enormous,  and  only  a  small 
part  of  the  centre  of  the  field  reasonably  sharp,  it  will  be  realized 
that  with  this  maximum  curvature  of  a  simple  lens  no  useful 
effect  would  be  obtained.  It  is  therefore  necessary  that  the  lens 
should  be  of  smaller  diameter  than  twice 
its  radius.  For  instance,  a  lens  of  \  inch  CT 
diameter  and  1  inch  focus  would  be  more 
practical  in  working.  To  merely  call  atten- 
tion to  this  matter  will  be  sufficient  to  show 
that  a  large  diameter  and  high  magnification 
cannot  be  associated  with  a  simple  lens. 

Special  reference  must  be  made  to  the 
Greenough  binocular  microscope,  which  is  a 
speciality  of  the  firm  of  Carl  Zeiss.  It  is  not  an  instrument 
that  is  suited  for  general  work,  being  principally  designed  for 
the  preparation  of  objects.  The  Porro  prism-erecting  system 
is  employed  for  the  body,  which  consists  of  a  combination  of 
two  incorporated  bodies  side  by  side,  and  two  special  objectives 
accurately  paired  and  mounted  at  such  an  angle  to  each  other 
that  they  both  embrace  exactly  the  same  portion  of  the  field. 
This,  it  will  be  noticed,  is  quite  different  from  the  ordinary 
Wenham  binocular  microscope,  in  which  a  single  objective  is 
used,  and  the  division  of  the  light  into  the  two  bodies  is  effected 
by  means  of  a  prism. 

Only  a  limited  range  of  objectives  can  be  applied  to  the 
Greenough  instrument ;  these  in  each  case  have  to  be  specially 
arranged  and  supplied  on  an  appropriate  carrier,  but  they  can 
be  had  in  a  range  of  magnifications  from  8  to  72  diameters. 

As  before  remarked,  the  Porro  prism  system  enables  the  object 
to  be  seen  the  right  way  up ;  but,  beyond  this,  a  beautiful  stereo- 
scopic effect  is  obtained — superior,  probably,  from  all  points  of 
view,  to  any  other  different  arrangement. 

Microscopists  who  have  been  accustomed  to  the  binocular  micro- 
scope, viewing  a  natural  history  specimen  through  the  Greenough 
binocular,  have  never  failed  to  express  astonishment  and  pleasure. 


40 


MODERN  MICROSCOPY 


Microscopes  for  Metallurgical  Work. 

Examination  of  metals  by  means  of  the  microscope  is  a  com- 
paratively modern  study,  but  there  is  probably  no  iron  or  steel 
works  of  standing  that  is  not  equipped  with  suitable  instruments 
both  for  observing  and  photographing.     By  means  of  the  micro- 


Fig.  17. — Simple  Pattern  of  Metallurgical  Microscope,  showing 

Adjustable  Stage. 

scope  much  information  regarding  both  the  chemical  constitution 
and  mechanical  properties  are  disclosed,  but  it  is  especially 
valuable  for  the  latter.  For  instance,  the  structure  of  steel  varies 
with  the  degrees  of  hardness  and  the  amount  of  heat  to  which 
it  has  been  subjected,  and  it  is  possible  readily  to  gain  definite 
information  concerning  the  suitability  of  the  metal  for  the  service 
to  which  it  is  to  be  put  by  means  of  the  microscope.  In  the 
manufacture  of  guns  any  defect  which  may  have  taken  place  in 


THE  MICKOSCOPE -STAND 


41 


jiiiiiiiiuiuuuui 


ttlK 


Fig.  18. 


the  heating  or  quenching  of  the  steel,  which  would  render  the  gun 
unsafe  or  unsatisfactory,  can  be  discovered  before  the  manufac- 
ture is  proceeded  with.  Engineers  can  detect  flaws,  blow-holes, 
defective  welds,  etc.,  at  an  early  stage,  and  avoid  the  trouble 
incident  to  the  use  of  imperfect  metal  in  the  finished  article. 

The  main  support  of  the  stage  is  carried  in  a  dovetailed  fitting, 
parallel  with  the  body  of  the  instrument,  and  can  be  raised  or 
lowered  by  means  of  a  rackwork  and  pinion. 

The  stage  itself  has  on  the  upper  surface  a  levelling  plate,  on 
which  the  specimen  for  examination  is  laid.  Three  screws  permit 
of  any  want  of  parallelism 
between  the  faces  of  the 
specimen  being  compensated 
for. 

To  illuminate  the  speci- 
men, a  vertical  illuminator, 
the  construction  and  use  of 
which  is  referred  to  on  p.  1 13, 
and  which  is  fitted  between 
the  objective  and  the  nose- 
piece,   is   employed    for 

medium  and  high-power  examinations.  The  lamp  and  bull's-eye 
have  to  be  placed  in  fixed  relation  to  this  vertical  illuminator, 
and  it  is  important  that,  once  the  illuminant  is  adjusted,  no 
further  movement  should  take  place ;  this  renders  obvious  the 
utility  of  the  rackwork  for  raising  and  lowering  the  whole  of  the 
mechanical  stage,  and  the  addition  of  a  fine  adjustment  to  this 
part  of  the  stage  becomes  an  added  convenience. 

The  use  of  this  class  of  microscope  is  by  no  means  restricted 
to  iron  and  steel  examination.  Similar  instruments  are  largely 
used  for  the  examination  of  brass,  in  addition  to  other  metals, 
and  a  large  quantity  of  alloys  and  compositions,  such,  for 
instance,  as  certain  material  of  which  billiard-balls  are  made. 

It  will  be  obvious  that  the  subjects  examined  are  opaque, 
and  the  usual  mirror  and  sub-stage  apparatus  are  dispensed 
with.  To  conduct  such  work  in  the  most  advantageous  manner, 
the  microscope  should  be  specially  designed  for  the  purpose  and 
kept  to  it.  Long  experience  produces  the  conviction  that  special 
work  of  such  a  nature  should  be  done  with  an  instrument  devised 


-Holder  for  Metallurgical 
Work. 


42 


MODEEN  MICEOSCOPY 


for  the  particular  purpose,  rather  than  by  attempting  to  make 
an  ordinary  instrument  serviceable.  So  extensive  has  this  work 
become,  that  all  leading  makers  construct  metallurgical  micro- 


Fig.  19. — Traveller's  Microscope. 
By  J.  Swift  and  Son. 

scopes.    The  patterns  vary,  but  the  main  features  will  be  gleaned 
from  the  illustration  of  one  of  these  stands  on  p.  40. 

For  some  purposes  a  holder  for  gripping  the  metal  under 
examination  has  been  found  advantageous,  and  this  is  particu- 
larly the  case  if  a  microscope  of  the  ordinary  type  is  available. 
There  are  several  different  types,  but  the  one  depicted  in  Fig.  18 


THE  MICEOSCOPE-STAND  43 

is  a  simple  one,  which  can  be  made  to  carry  a  fairly  large  piece 
of  metal  between  its  adjustable  jaws. 

If  an  ordinary  microscope  is  employed  for  metallurgy,  it  will 
be  found  convenient  to  use  the  sub-stage  as  the  object-carrier 
instead  of  the  stage. 

The  vertical  illuminator  will  generally  give  the  requisite  length 
to  bring  the  objective  into  focus  if  the  specimen  be  carried  on 
the  sub-stage,  and  the  latter  racked  up  nearly  as  high  as  it  will 
go.  It  will  be  seen  by  this  arrangement  that  the  light  can  be 
adjusted  to  the  vertical  illuminator,  and  the  focussing  done  by 
means  of  the  sub-stage ;  the  metal-holder  previously  referred  to, 
or  a  special  plate  with  a  fitting  to  go  in  the  sub-stage,  will 
thus  enable  occasional  work  to  be  done. 

For  low-power  work  with  the  1-inch  objective  or  less  mag- 
nification, the  ordinary  bull's-eye  or  stand  condenser,  together  with 
a  parabolic  reflector  (referred  to  on  pp.  110  and  113),  are  used. 
Sometimes  the  latter  is  used  in  conjunction  with  an  additional 
reflecting  plate,  arranged  in  a  manner  devised  by  Sorby. 

It  requires  to  be  borne  in  mind  that  records  have  frequently 
to  be  made  photographically  of  metallurgical  specimens  after 
visual  examination,  and  due  consideration  must  be  given,  in  the 
selection  of  an  instrument,  to  its  suitability  for  the  combined 
purposes ;  particularly,  when  it  is  set  horizontally,  there  must 
be  no  tendency  to  unsteadiness  ;  it  must  be  equally  firm  in  all 
working  positions. 

Portable  or  Traveller's  Microscope. 

A  lover  of  the  microscope  feels  lost  on  his  holidays,  or  when 
travelling,  without  his  microscope,  and  numerous  attempts  have 
been  made  to  produce  instruments  that  shall  have  the  satis- 
factory working  qualities,  and  yet,  by  folding  and  detaching 
parts,  occupy  a  minimum  of  space.  The  necessity  of  such  in- 
struments has  been  appreciated  by  the  numerous  investigators 
of  tropical  diseases,  and  this  type  of  microscope  has  played  no 
inconsiderable  part  in  this  class  of  work  and  in  clinical  and 
medical  observations.  From  the  amateur's  point  of  view,  one 
of  the  most  successful  instruments  is  that  by  Swift  and  Son, 
shown  in  Fig.  19,  which  is  fitted  into  a  case,  with  objectives, 


44  MODERN  MICROSCOPY 

eyepiece,  and  several  useful  accessories,  measuring  9  x  3J  x  3 
inches.  Excellent  models  are  also  made  by  C-  Baker,  R.  and 
J.  Beck,  Leitz,  and  W.  Watson  and  Sons,  the  last  two  named 
also  offering  instruments  of  heavier  type,  rendered  especially 
compact  for  high-power  work. 

Penological  Microscopes. 

The  examination  of  rocks  and  minerals  has  become  a  depart- 
ment of  such  great  prominence  that  a  larger  number  of  special 
microscopes  have  been  introduced  for  this  specific  purpose  during 
the  last  decade.  Swift  and  Son,  who  are  specialists  in  this 
particular  class  of  instrument,  offer  no  less  than  seven  different 
models  and  instruments  of  excellent  design,  all  having  distinc- 
tive features,  and  others  are  manufactured  by  R.  and  J.  Beck, 
Leitz,  W.  Watson  and  Sons,  and  Zeiss. 

With  such  a  variety,  it  is  difficult  to  indicate  the  essentials 
without  knowing  the  precise  work  that  is  to  be  undertaken. 
Very  simple  arrangements  will  suffice  for  occasional  work,  while 
every  elaboration  possible  is  necessary  for  the  more  gregarious 
worker.  Perhaps  it  will  be  well  to  mention  the  way  in  which 
the  ordinary  microscope  may  be  employed  in  petrology  in  order 
to  indicate  what  is  necessary.  First,  the  stage  should  be  a 
concentric  rotating  one,  with  the  circumference  divided  to 
degrees,  reading  by  a  vernier  or  pointer,  so  that  the  crystal 
under  examination  may  be  measured  and  its  angle  ascertained. 
The  polarizer,  having  a  large-size  prism,  must  be  used  beneath 
the  stage,  and  the  rotating  flange  should  be  divided  and  have 
an  indicator  for  reading. 

For  a  certain  class  of  work,  a  condenser  of  large  numerical 
aperture  would  require  to  be  adaptable  to  the  polarizer.  The 
analyzer  can  be  fitted  above  the  eyepiece  also  with  a  divided 
circle.  The  quartz  wedge  can  advantageously  be  placed  in  a 
carrier  between  the  objective  and  the  nosepiece,  and  the  Ber- 
trand's  lens  for  observing  the  interference  of  figures  of  crystals 
screwed  to  the  lower  end  of  the  draw-tube.  A  special  eyepiece 
would  have  to  be  provided  having  cross-webs  fitted  to  the 
diaphragm,  and  slots  should  be  made  either  in  the  eyepiece 
or  immediately  above  it  to  receive  the  undulation  plate,  micro- 
meter, etc. 


THE  MICROSCOPE-STAND 


45 


Fig.  20. — Petrological  Microscope. 
By  Swift  and  Son  ;  designed  by  Mr.  Allan  Dick. 


46 


MODERN  MICROSCOPY 


Now,  all  these  fittings  can  be  applied  to  an  ordinary  microscope 
at  any  time,  but  where  penological  study  is  regularly  undertaken, 
it  is  far  better  to  be  equipped  with  an  instrument  built  for  the 


Fig.  21. — Stand  VI.,  by  Leitz. 
A  very  inexpensive  microscope,  suitable  for  preparing  and  mounting  specimens. 

specific  purpose,  with  all  these  conveniences  incorporated  in  a 
manner  that  enables  full  control  and  rapid  manipulation  to  be 
exercised. 


THE  MICROSCOPE-STAND  47 

The  microscope  illustrated  and  designed  by  Mr.  Allan  Dick  has 
been  a  standard  model  for  many  years.  The  striking  feature  in 
it  is  that  the  stage  is  fixed,  and  instead  of  rotating  the  specimens, 
the  polarizer  and  analyzer  prisms  with  the  eyepiece  are  made  to 
revolve  together.  It  is  not  universally  agreed  that  this  arrange- 
ment is  the  best,  but  it  has  been  much  favoured,  and  it  will  be 
at  once  seen  that  the  trouble  connected  with  the  exact  centring 
of  the  rotating  stage  is  obviated  by  this  means. 

The  microscope  in  petrology  is  dealt  with  in  a  separate 
chapter  (p.  219),  and  reference  should  be  made  to  this  for 
further  information. 


Microscopes  for  Preparing  and  Mounting. 

A  plain  substantial  microscope-stand,  which  the  user  would 
have  no  compunction  in  soiling,  is  of  inestimable  value  in  the 
preparing  and  mounting  of  micro-slides,  and  a  suitable  one  can 
be  obtained  for  such  a  small  sum  that  it  is  a  pity  that  instru- 
ments designed  for  work  of  a  higher  grade  should  be  employed 
for  the  purpose.  We  figure  on  p.  46  an  illustration  of  the 
type  of  microscope  that  will  be  found  very  serviceable  for  this 
purpose.  When  we  mention  that  the  cost  of  this  is  only  20s., 
that  it  is  provided  with  rackwork  and  pinion,  and  that  it  has  the 
standard  screw  for  objectives,  and  receives  eyepieces  of  Conti- 
nental diameter,  it  will  probably  need  no  further  recommendation. 

The  Ultra-Microscope. 

This  interesting  instrument  was  invented  by  Dr.  H.  Siedentopf , 
a  member  of  the  scientific  staff  of  the  Jena  Glass  Works,  and  is 
manufactured  by  Carl  Zeiss.  It  renders  visible  particles  which 
are  beyond  the  limits  of  vision  with  the  ordinary  microscope, 
even  with  the  highest  powers,  and  are  hence  described  as  ultra- 
microscopical. 

It  has,  of  course,  long  been  known  that  whilst  there  is  a 
fairly-well-defined  limit  of  resolving  power — in  other  words, 
a  minimum  distance  beyond  which  two  or  more  particles  or 
elements  of  structure  cannot  be  seen  apart — there  is  no  such 
limit  for  the  size  of  the  separate  particles  themselves,  at  any 


48  MODERN  MICROSCOPY 

rate  when  they  are  placed  in  a  strong  light  against  a  dark  back- 
ground ;  for  under  these  conditions  the  smallness  of  the  reflect- 
ing surface  of  the  particles  can  be  compensated  for  by  the 
brightness  of  the  illumination  in  much  the  same  way  as  minute 
particles  of  floating  dust  become  visible  in  the  path  of  a  ray  of 
sunlight  through  an  otherwise  shaded  room. 

The  effect  is  obtained  in  a  most  ingenious  manner  by  taking- 
advantage  of  total  reflection.  The  condenser  (a  dry  one)  is 
arranged  at  right  angles  to  the  optical  axis  of  the  microscope,  so 
that  its  concentrated  light  enters  the  slide  containing  or  forming 
the  object  laterally — through  the  edge.  None  of  this  light  can 
emerge  into  air  through  the  upper  surface  of  the  slide,  as  this 
would  demand  a  refractive  index  of  1*414 — lower  even  than  that 
of  fluorite.  The  light  diffused  by  small  particles  is,  therefore, 
the  only  light  that  reaches  the  eye,  and  the  latter  will  therefore 
be  sensitive  to  its  utmost  limit. 

But  it  is  further  necessary  to  limit  the  illumination  to  particles 
within  the  depth  of  focus,  as  otherwise  the  light  scattered  by 
particles  not  in  focus  would  form  a  luminous  background.  This 
object  is  attained  by  focussing  the  source  of  light — generally  the 
electric  arc — on  an  adjustable  spectroscope- slit,  then  focussing 
the  slit  in  the  object. 

By  the  use  of  this  refined  method  Dr.  Siedentopf  proved 
by  direct  observation  that  the  beautiful  red  glass,  known  as 
gold-ruby  glass,  contains  small,  detached  particles  of  gold  of 
extremely  small  size.  By  computing  the  cubic  space  under 
observation,  counting  the  particles  within  that  space,  and 
ascertaining  by  analysis  the  percentage  of  gold  in  the  glass 
under  examination,  it  was  found  that  the  particles  were  equal  in 
weight  to  tiny  cubes  of  gold  having  sides  equal  to  from  one-sixth 
to  five  one-millionths  of  an  inch.  Though  almost  inconceivably 
minute,  the  particles  are  still  composed  of  a  great  number  (at 
least  10,000)  of  molecules.  These  latter  must,  therefore,  still  be 
considered  as  invisible. 

By  its  means  very  minute  particles  of  other  origin  can  also 
be  demonstrated — such,  for  instance,  as  the  flagellar  of  certain 
bacteria. 

Reference  is  made  to  this  subject,  not  by  reason  of  its 
importance  to  the  elementary  worker,  but  because  it  shows  that 


THE  MICROSCOPE-STAND  49 

the  possibilities  of  the  microscope  have  not  yet  reached  their 
limit ;  and,  although  at  present,  work  with  this  instrument  is  so 
specialized  that  it  does  not  come  within  the  range  of  the  ordinary 
worker,  future  developments  may  take  place  on  similar  lines 
which  may  make  it  more  generally  useful. 

Modified  effects  of  the  nature  indicated  are  obtained  by  the  use 
of  a  recently  introduced  illuminator,  known  as  the  '  immersion 
paraboloid,'  the  '  paraboloidal  condenser,'  etc.  These  are  fully 
described  on  p.  105.  They  are  sometimes  inaccurately  described 
as  '  ultra-microscopes,'  but  the  construction  is  quite  dissimilar, 
and  the  titles  should  not  be  indifferently  applied. 


CHAPTER  II 
OPTICAL  CONSTRUCTION 

Preliminary  Note. 

In  the  former  part  of  this  book  we  have  dealt  exclusively  with 
the  stand,  or  mechanical  means  of  employing  the  optical  system 
and  accessories  ;  and  important  as  it  is  that  those  details  shall 
be  very  efficient,  it  is,  if  anything,  still  more  so  that  the  eye- 
pieces, objectives,  and  illuminating  apparatus  shall  be  of  the 
most  perfect  description,  properly  adapted  and  intelligently 
employed,  for  on  the  optical  combinations  depend  the  results 
that  are  to  be  obtained  with  the  stand  ;  and  although  care  and 
trouble  may  enable  a  person  to  use  a  bad  stand,  no  good  stand 
can  ever  compensate  in  any  way  for  bad  objectives.  It  requires 
constant  practice  and  a  long  apprenticeship  to  learn  to  use  the 
microscope  to  the  utmost  advantage.  Every  subject  of  examina- 
tion calls  for  special  manipulative  treatment  if  it  is  to  be  correctly 
understood  and  appreciated.  Experience  alone  can  guide  in 
obtaining  the  best  result  under  varied  circumstances,  and  that 
experience  must  be  based  on  a  knowledge  and  understanding  of 
correct  methods  in  working. 

Definitions. 

Some  of  the  following  terms  will  be  made  use  of  in  this  book, 
and  are  constantly  met  with  in  literature  on  microscopical 
subjects ;  a  brief  explanation  of  them  may  therefore  prove  of 
service. 

'  Achromatic  Correction. — Owing  to  the  relatively  greater  dis- 
persive power  of  flint-glass  (containing  lead  or  other  heavy 
metals)  as  compared  with  crown-glass,  it  is  possible  to  produce 

50 


OPTICAL  CONSTRUCTION  51 

a  combination  of  a  convex  lens  of  crown-glass  with  a  concave 
lens  of  flint,  which  collects  rays  like  a  simple  convex  lens,  but 
which  unites  two  different  colours  in  the  same  focal  point,  thus 
in  a  great  measure  correcting  the  chromatic  aberration. 

Aplanatism. — A  freedom  from  spherical  aberration  (see  below). 

Apochromatic  Correction. — The  highest  attainable  correction 
of  microscope  objectives,  comprising  the  correction  of  spherical 
aberration  for  all  colours,  and  the  union  of  three  different  colours 
in  one  focus,  thereby  eliminating  the  secondary  spectrum. 

Chromatic  Aberration. — White  light  is  the  composite  effect  of 
a  continuous  range  of  colours,  passing  from  red,  through  yellow, 
green,  and  blue  to  violet  (see  Spectrum).  All  transparent  media 
have  different  refractive  indices  for  these  different  colours,  and, 
as  a  consequence,  after  their  passage  through  a  simple  lens  the 
rays  do  not  unite  at  one  focal  point.  The  red  rays,  being  the 
least  refrangible  and  bending  to  the  smallest  extent,  unite  at 
the  farthest  distance  from  the  lens;  the  orange  and  green  rays 
unite  at  points  closer  to  the  lens ;  while  the  violet  rays  come  to 
a  focus  at  a  point  nearest  to  the  lens.  The  confusion  of  dif- 
ferent coloured  images  resulting  from  this  dispersion  is  termed 
'  chromatic  aberration.' 

Chromatic  Over-Correction. — A  term  used  when  a  lens  brings 
yellow  or  even  orange  rays  to  the  shortest  focus  and  best 
correction. 

Chromatic  Under-Correction. — A  term  applied  to  a  lens  when 
rays  towards  the  blue  end  of  the  spectrum  are  best  corrected. 
Thus  a  photographic  lens  is  visually  under-corrected. 

Diaphragm. — This  is  generally  understood  in  optical  instru- 
ments to  be  a  circular  opening  in  a  plate  that  is  used  to  cut  off 
the  marginal  portions  of  a  beam  of  light,  and  in  this  sense  is 
referred  to  in  this  book.  The  diaphragm  is  often  improperly 
called  a  stop. 

Diffraction  Spectra. — If  we  look  through  a  finely  ruled  grating 
at  a  gas  or  candle  flame,  we  shall  see  a  large  number  of  images 
of  that  flame  having  the  colours  of  the  spectrum.  This  effect  is 
due  to  diffraction.  In  the  microscope,  objects  having  fine  and 
regularly  spaced  markings  diffract  the  light  in  a  similar  manner, 
the  resulting  diffraction  spectra  being  plainly  visible  at  the  back 
of  the  objective.     According  to  the  Abbe  theory  of  microscopic 


52  MODERN  MICROSCOPY 

vision,  these  diffraction  spectra  determine  the  character  of  the 
image  seen,  the  latter  becoming  less  like  the  real  structure  when 
the  number  of  diffraction  spectra  admitted  by  the  objective  is 
reduced,  a  faithful  representation  of  the  object  being  obtainable 
only  when  all  diffracted  light  of  sensible  brightness  is  admitted. 
A  further  note  on  this  interesting  subject  by  the  late  Dr.  G.  John- 
stone Stoney,  F.R.S.,  will  be  found  on  p.  134. 

Female  and  Male  Screws. — The  former  is  a  threaded  fitting 
which  receives  a  screw,  and  the  latter  a  screw  which  goes  into 
the  female  fitting.  In  the  case  of  a  bolt  and  nut  the  former 
would  have  the  male,  and  the  latter  the  female  screws. 

N.A.,  abbreviation  for  'numerical  aperture.'     See  p.  62. 

Negative  Eyepiece. — This  is  an  eyepiece  for  examining  an  image 
formed  at  the  diaphragm  set  between  the  two  component  plano- 
convex lenses.  The  Huyghenian  is  the  best-known  form  of 
negative  eyepiece. 

O.I.,  abbreviation  for  '  optical  index.'     See  p.  68. 

Positive  Eyepiece. — This  is  an  eyepiece  for  examining  an  image 
situated  beyond  the  field  lens.  It  can  consequently  be  used  as  a 
magnifying-glass,  etc. 

Refractive  Index. — When  a  ray  of  light  passes  obliquely  from 
one  medium  into  another  of  different  density,  the  path  of  that 
ray  is  bent  or  altered  in  its  course.  According  to  the  law  of 
refraction,  there  is  a  constant  ratio  for  any  given  two  media 
between  the  sine  of  the  angle  of  incidence  (being  the  angle 
included  between  the  incident  ray  in  the  first  medium  and  the 
perpendicular)  and  the  sine  of  the  angle  of  refraction,  or  of  the 
angle  included  between  the  ray  after  refraction  and  the  same 
perpendicular.  The  numerical  value  of  this  ratio  for  a  ray 
passing  from  air  into  a  medium  is  called  the  refractive  index  of 
the  medium. 

Secondary  Spectrum. — In  an  achromatic  lens  the  chromatic 
aberration  is  corrected  for  the  brightest  (yellow  or  green)  rays 
of  the  spectrum,  and  the  pronounced  colour  shown  by  uncor- 
rected lenses  is  in  consequence  removed.  A  stricter  examination, 
however,  shows  that  rays  of  a  different  colour  are  not  brought 
to  the  same  focus,  for  owing  to  the  fact  that  flint-glass,  as  com- 
pared with  crown-glass,  disperses  the  more  refrangible  rays 
relatively  too   much,   and   the   least   refrangible   relatively  too 


OPTICAL  CONSTRUCTION  53 

little,  a  peculiar  secondary  spectrum  results  from  the  achromatic 
combination,  the  rays  corresponding  to  the  brightest  apple- green 
part  of  the  ordinary  spectrum,  being  very  closely  united  and 
focussed  nearest  the  combination,  whilst  the  other  colours  focus 
at  increasing  distances  in  pairs,  yellow  being  united  with  dark 
green,  orange  with  blue,  red  with  indigo.  The  composite  effect 
of  these  colours  is  best  seen  with  oblique  light,  causing  dark 
objects  to  have  apple-green  borders  on  one  side  and  purple  ones 
on  the  other. 

Semi-Apochromatic  Correction. — In  achromatic  microscope  ob- 
jectives of  the  older  type,  chromatic  defects  that  are  worse  than 
the  secondary  spectrum  are  caused  by  spherical  aberration  of  the 
coloured  rays,  the  spherical  aberration  being  corrected  for  the 
brightest  part  of  the  spectrum  only.  Objectives  made  entirely 
of  glass,  and  therefore  showing  the  secondary  spectrum,  are 
called  semi-apochromatic  when  the  spherical  aberration  is  cor- 
rected practically  for  all  colours. 

Spectrum. — A  band  of  colours  produced  by  the  splitting  up  of 
white  light  by  means  of  a  prism.  The  order  of  the  colours  is : 
Eed,  orange,  yellow,  green,  blue,  indigo,  violet. 

Spherical  Aberration. — Rays  of  light  passing  through  the 
marginal  portion  of  a  lens  come  to  a  focus  nearer  to  the  lens 
itself  than  those  rays  which  pass  through  the  centre  of  the 
lens,  and  the  interval  between  the  focal  points  of  rays  which 
pass  through  the  marginal  and  the  central  parts  of  that  lens  is 
the  spherical  aberration.  In  compound  lenses  this  spherical 
aberration  can  be  corrected  for  one  or  more  special  rays,  and  a 
lens  so  corrected  is  called  aplanatic.  It  is  only  truly  aplanatic 
for  the  particular  rays  for  which  it  has  been  accurately  corrected. 

Spherical  Over-Correction  is  present  when  a  lens  unites  the 
marginal  rays  at  a  greater  distance  than  the  central  rays. 
Spherical  under-correction  is  indicated  when  the  marginal  rays 
focus  closer  to  the  lens  than  the  central  ones. 

Spherical  Zones. — In  objectives  of  considerable  aperture  the 
intermediate  rays  may  show  decided  spherical  aberration,  although 
the  central  and  marginal  rays  are  united.  This  defect  is  meant 
when  spherical  zones  are  spoken  of.  The  degree  to  which 
spherical  zones  are  corrected  determines  chiefly  how  large  a 
cone  of  illumination,  and  how  deep  an  eyepiece  an  objective 


54  MODERN  MICROSCOPY 

will  bear  before  '  breaking  down.'  A  high  degree  of  correction 
for  spherical  aberration  and  spherical  zones  must  accompany  the 
reduction  of  chromatic  defects  before  terms  such  as  '  semi- 
apochromatic,'  and  especially  '  apochromatic,'  can  be  applied  to 
a  lens. 

Stoj). — In  an  optical  instrument  this  is  a  means  of  obstructing 
the  passage  of  the  central  portion  of  a  beam  of  light. 

MAGNIFYING  POWER, 

It  has  been  previously  stated  that  magnifying  power  is  not 
dependent  on  the  size  of  the  instrument.  Given  suitable  eye- 
pieces and  objectives,  the  same  magnification  may  be  obtained 
on  a  small  microscope  as  on  a  large  one.  It  is  entirely  pro- 
duced by  the  two  optical  parts — the  objective,  and  the  eyepiece, 
or  ocular.  Under  the  head  of  '  Objectives '  in  the  English 
makers'  catalogues  it  will  be  noticed  that  the  powers  are 
expressed  as  1-inch,  ^-inch,  ^-inch,  etc.  The  figures  do  not 
indicate  the  distance  at  which  the  lenses  focus  on  the  object, 
but  are  intended  to  approximately  convey  the  equivalent  focus, 
and  thereby  the  actual  magnifying  power  of  the  objective.  To 
understand  this  description,  imagine  an  objective  to  be  placed 
so  as  to  form  an  image  of  an  object  at  10  inches  from  its  back 
lens.*  Then,  an  objective  which,  when  so  placed,  formed  an 
image  on  the  screen  which  was  ten  times  (diameters)  the  real 
size  of  the  object  would  be  described  as  a  1-inch  objective  ;  one 
which  formed  an  image  twenty  times  the  size  of  the  object  would 
be  called  a  J-inch  objective,  and  in  general  the  result  given,  when 
the  magnification  of  the  image  formed  on  the  screen  is  divided 
into  ten,  is  what  is  spoken  of  as  the  focal  length  of  the  objective; 
also  the  equivalent  focus  of  an  objective  divided  into  ten  gives 
its  magnifying  power — thus  a  2-inch  should  magnify  5,  a  ^-inch 
40,  and  a  -J-inch  80,  diameters.  This  is  termed  the  initial  mag- 
nifying power  of  an  objective. 

*  The  above  plan  will  be  sufficiently  accurate  for  experimental  purposes, 
but,  strictly  speaking,  it  is  the  equivalent  focus  of  the  objective  which 
determines  its  magnifying  power,  and  in  order  to  obtain  exact  results  the 
measurements  should  be  taken  from  the  upper  focal  plane  of  the  objective. 
The  optical  tube-length  should  also  be  reckoned  in  like  manner,  and  this  may 
generally  be  assumed  to  be  from  \  inch  to  1  inch  longer  than  the  mechanical 
t  ube-length  or  the  length  of  the  body  of  the  microscope. 


OPTICAL  CONSTRUCTION  55 

The  foci  of  German  objectives  are  usually  expressed  in  milli- 
metres, 250  millimetres  (about  9\~  inches)  being  taken  as  the 
normal  vision  distance,  and  the  focal  length  of  the  objective 
divided  into  250  gives  the  initial  magnifying  power.*  Thus  a 
3-millimetre  objective  should  have  an  initial  power  of  83 J,  and  a 
4-millimetre  of  62J,  diameters,  and  so  on. 

The  image  formed  by  the  objective  is  again  magnified  by  the 
eyepiece.  Unfortunately,  the  latter  is  rarely  marked  with  its 
magnifying  power,  the  general  rule  being  to  call  the  different 
powers  by  the  letters  A,  B,  C,  D,  etc.,  or  1,  2,  3,  4,  etc.  This  is 
not  very  intelligible,  and  it  would  be  far  better  either  to  express 
their  equivalent  focus,  as  in  the  case  of  objectives,  or  to  have 
the  magnifying  power  in  diameters  marked  on  the  cap.  We  will 
take  it  that  the  'A'  eyepiece  yields  a  magnification  of  5  diameters. 
When  this,  therefore,  is  used  in  conjunction  with  the  1-inch 
objective  on  a  10-inch  tube-length,  which  according  to  the  rules 
previously  given  would  produce  a  magnification  of  10  diameters, 
the  resultant  combined  power  is  50 — that  is,  the  powers  of 
the  objective  and  eyepiece  multiplied  together.  The  method 
of  estimating  the  power  with  short  tube  -  lengths  is  referred 
to  on  p.  72. 

OBJECTIVES. 

For  our  purpose  we  shall  divide  the  subject  of  objectives  into 
two  classes — (1)  the  apochromatic,  and  (2)  the  achromatic.  The 
immersion  objective  which  may  belong  to  either  of  these  two 
classes  is  referred  to  separately.  As  in  our  remarks  on  objec- 
tives we  shall  constantly  use  the  two  terms,  we  will  describe 
their  reference. 

Apochromatic  Objectives. — The  introduction  of  these  objec- 
tives by  the  firm  of  Carl  Zeiss,  of  Jena,  Germany,  about  twenty - 
five  years  ago,  placed  the  science  of  microscopic  optics  on  a 
far  higher  level  than  had  hitherto  been  attained,  and  as  a 
result  many  of  the  traditional  modes  of  working  were  altered. 
Greater  precision  has  been  necessitated  in  the  microscope-stand, 
and  the  provision  of  sub-stage  condensers  of  corresponding 
optical  quality  to  the  objectives  has  been  essential.  Professor 
Abbe  and  Dr.  Schott  were  granted  a  subsidy  by  the  Prussian 

*  See  note  on  p.  54. 


56  MODEEN  MICEOSCOPY 

Government  with  a  view  to  the  promotion  of  optical  research, 
and  aided  by  this  they  were  able  to  produce  several  varieties  of 
new  optical  glass.  The  employment  of  these  new  glasses  in 
conjunction  with  fluorite,  based  upon  the  careful  and  elaborate 
calculations  of  Professor  Abbe,  resulted  in  the  production  of 
apochromatic  objectives.  In  these  lenses,  aberrations  which 
were  inherent  in  the  older  systems  were  eliminated  or  minimized 
— that  is  to  say,  the  secondary  spectrum  was  practically  removed, 
and  spherical  aberration  was  very  perfectly  corrected  for  all 
colours.  The  objective,  therefore,  produced,  to  all  intents  and 
purposes,  a  colourless  image.  Higher  apertures  were  obtainable, 
and  in  consequence  of  the  improved  corrections,  accompanied  by 
greater  brilliance  of  the  field,  the  use  of  eyepieces  of  high  power 
was  rendered  permissible  and  advantageous. 

The  new  kinds  of  glass  were  placed  at  the  disposal  of  opticians 
throughout  the  world,  and  apochromatic  objectives  have  been 
since  manufactured  by  other  firms,  whose  productions  compare 
favourably  with  the  best  of  the  originators'  lenses.  The  apo- 
chromatic objectives  by  Zeiss  have  their  equivalent  focus  engraved 
in  millimetres,  and  it  is  becoming  usual  for  the  same  method  to 
be  applied  to  other  objectives  also.  The  initial  magnifying  power 
of  such  lenses  is  ascertained  by  dividing  the  equivalent  focus  in 
millimetres  into  250.  Thus,  a  lens  with  an  equivalent  focus  of 
2*5  millimetres  would  have  an  initial  magnifying  power  of  100 
diameters. 

Special  eyepieces,  termed  '  compensating  oculars,'  are  necessary 
when  using  the  apochromatic  objectives.  They  will  be  found 
described  on  p.  84. 

Achromatic  Objectives. — All  objectives  that  are  not  actually 
comprised  in  the  apochromatic  category — that  is,  in  which  the 
secondary  spectrum  is  not  eliminated — are  included  under  this 
heading.  So  far  as  the  principal  opticians  are  concerned,  it 
comprehends  a  better  class  of  objectives  than  it  did  at  the  period 
when  apochromatic  lenses  were  introduced.  By  the  use  of  the 
new  optical  glasses  previously  referred  to,  and  in  consequence 
of  keen  competition  amongst  manufacturers,  many  achromatic 
objectives,  tending  towards  apochromatism,  have  been  made. 
Several  of  these  are  so  well  corrected  that  in  some  instances 
they  vie  with  the  apochromatics  in  performance. 


OPTICAL  CONSTRUCTION  57 

The  class  has  consequently  arisen  which  has  been  referred  to 
under  the  generic  term  of  semi-apochromatic.  Many  of  these 
lenses  are  made  in  such  perfection  as  to  be  superior  even  in  some 
features  to  the  real  apochromatics.  Some  of  the  lenses  in  Watson 
and  Sons'  new  series  of  '  Holoscopic '  objectives,  which  require  to 
be  used  with  over-corrected  eyepieces  of  the  compensating  type, 
are  especially  free  from  spherical  aberration.  Messrs.  Swift 
and  Son,  of  London,  in  their  series  of  pan-aplanatic  objectives, 
produce  beautiful  results  ;  also  C.  Eeichert,  of  Vienna,  and  Leitz, 
of  Wetzlar,  make  objectives  that  are  worthy  of  special  con- 
sideration. Beyond  these  there  are  excellent  series  of  lenses 
made  by  all  the  microscope  manufacturers  which  meet  the  re- 
quirements of  the  ordinary  amateur  in  a  most  efficient  manner 
— in  fact,  the  general  quality  of  such  objectives  is  superior  to 
that  which  obtained  in  the  so-called  best  lenses  of  a  few  years  ago. 

Achromatic  versus  Apochromatic  Objectives. — In  view  of  the 
foregoing  facts,  it  will  be  well  to  consider  which  series  of  objec- 
tives should  be  selected  for  specific  work.  It  has  to  be  re- 
membered that  apochromatic  objectives  are  very  expensive,  and, 
generally  speaking,  are  beyond  the  reach  of  the  ordinary  amateur, 
who  usually  takes  up  microscopy  without  special  scientific  aims, 
and  excepting  to  a  trained  critical  eye  they  would  not  be  found 
to  possess  the  extraordinary  merit  that  is  claimed. 

The  question  naturally  occurs,  Is  it  worth  while  to  incur  the 
great  cost  which  is  involved  in  the  purchase  of  apochromatic 
objectives  ?  No  decided  opinion  can  be  given  without  a  full 
knowledge  of  the  scope  of  the  work  which  is  to  be  undertaken, 
and  as  from  the  nature  of  things  it  is  impossible  at  its  inception 
to  tell  the  extent  to  which  research  may  be  carried,  the  difficulty 
of  giving  advice  is  increased.  Generally  speaking,  it  may  be 
stated  definitely  that  for  the  ordinary  work  of  the  amateur,  the 
so-called  '  students'  series '  of  lenses  will  be  found  to  give  all 
the  pleasure  and  satisfaction  that  are  to  be  derived  from  the 
examination  of  Nature's  small  things,  without  attempting  to 
obtain  apparently  impossible  results  or  to  detect  structure  not 
hitherto  discovered.  The  man  who  definitely  equips  himself  for 
original  research  cannot  afford  to  have  less  than  the  very  best 
means  which  modern  optical  skill  can  afford  him,  and  from  the 
point  of  view  of  actual  supremacy  the  apochromatics  must  then 


58  MODERN  MICROSCOPY 

be  chosen  ;  but  he  would  be  limiting  his  possibilities  in  practi- 
cally no  degree  whatever  by  having  lenses  carefully  selected 
from  those  previously  referred  to  under  the  title  of  semi-apochro- 
matics. 

It  should  be  remembered  that  the  reduction  of  spherical  zones 
enables  a  higher  power  of  eyepiece  to  be  employed  with  an 
objective  than  would  otherwise  be  possible,  and  it  is  due  to  this 
quality  that  the  apochromatic  objectives  have  been  especially 
valuable. 

A  series  of  compensating  eyepieces  is  specially  designed  to 
work  with  them,  having  powers  varying  from  2  to  27  diameters. 
Supposing,  therefore,  we  were  working  with  a  J-inch  apochromat 
having  an  initial  power  of  40  diameters,  with  a  10-inch  tube- 
length,  we  could  by  means  of  the  searcher  eyepiece  (  x  2)  obtain 
a  magnification  of  80  diameters,  and  by  using  intermediate 
powers  of  eyepieces  up  to  the  x  27,  produce  any  magnification 
that  might  be  desired  from  80  to  1,080  diameters  (J-inch  initial 
power  of  40  x  27  eyepiece  power  =  1,080). 

Further,  these  special  eyepieces  are  all  designed  to  work  in 
the  same  focal  plane  at  the  tube-length  for  which  the  eyepieces 
and  objectives  are  designed,  with  the  result  that  practically  very 
little  refocussing  is  necessary  on  the  exchange  of  an  eyepiece 
during  an  observation.  By  this  means  the  magnification  with  a 
low-power  objective  having  a  long  working  distance  and  a  fairly 
high  N.A.  for  its  power,  as  possessed  by  all  of  the  apochromats 
of  Messrs.  Zeiss's  manufacture,  can  be  gradually  increased,  and 
the  advantage  gained  is  one  for  which  many  microscopists 
sighed  before  the  days  of  apochromats — namely,  a  wide  range 
of  magnifying  power  and  great  working  distance.  For  many 
classes  of  work  this  convenience  is  very  great ;  but  it  must  not  be 
forgotten  that  medium-  and  low-power  eyepieces  are  desirable  for 
general  work,  that  it  is  advisable  to  use  such  high-power  eyepieces 
for  occasional  reference  or  for  testing  purposes  onty,  and  that  if 
high  magnification  is  necessary  it  is  always  to  be  insisted  that  a 
suitable  high-power  objective  with  a  medium-  or  low-power  eye- 
piece is  the  only  satisfactory  means  of  working. 

Some  of  the  best  of  the  achromatic  objectives,  to  which 
reference  has  already  been  made,  will  stand  as  high  an  eyepiece 
power  as  the  apochromats,  but  generally  they  do  not   advan- 


OPTICAL  CONSTRUCTION  59 

tageously  bear  anything  higher  than,  say,  up  to  10  or  12 
diameters,  and  usually  not  in  the  perfect  manner  that  the 
apochromats  do.  The  Huyghenian  eyepieces  that  are  used  with 
the  achromatics  are  very  rarely  designed  to  work  one  after  the 
other  in  the  same  focal  plane,  with  the  result  that  it  is  necessary 
to  refocus  every  time  the  eyepieces  are  exchanged,  and  the 
higher  the  power  of  the  eyepiece  that  is  employed  the  closer  will 
the  objective  work  to  the  object. 

Magnifying  power,  however,  is  not  the  only  feature  to  be 
considered  with  regard  to  an  objective ;  there  must  be  a  power 
of  delineating  fine  detail.  This  latter  quality  is  dependent  on 
the  numerical  aperture  of  the  objective,  referred  to  on  p.  62. 

Immbesion  Objectives. — In  using  these  objectives  a  film  of  a 
specified  fluid  is  interposed  between  the  front  lens  of  the  objec- 
tive and  the  cover-glass  of  the  object  under  examination,  so  that 
continuity  is  established  between  them.  There  are  two  media 
that  are  in  regular  use,  viz.,  water  and  cedar-wood  oil.  Others, 
including  glycerine  and  mono-bromide  of  naphthalin,  are,  how- 
ever, occasionally  employed.  It  may  be  taken  that  when  a  lens 
is  referred  to  as  a  '  homogeneous  or  immersion '  objective, 
cedar-wood  oil,  or  a  mixture  of  which  that  oil  is  the  principal 
ingredient,  known  as  '  immersion  oil,'  is  the  correct  medium  for 
using  with  that  objective.  The  refractive  index  of  cedar-wood 
oil  is  about  1*52,  and  practically  the  same  as  crown-glass  ;  conse- 
quently, when  it  is  used  for  immersion  purposes  it  has  the  effect 
of  rendering  the  cover-glass  part  of  the  objective. 

The  question  naturally  arises,  What  advantage  is  gained  by 
the  use  of  an  immersion  medium  ?  In  reply,  it  may  be  briefly 
stated  that  the  resolving  power  of  the  objective,  the  brilliance  of 
the  image,  and  the  working  distance,  are  all  increased. 

It  is  a  well-known  law  that  rays  passing  from  a  rarer  to  a 
denser  medium  are  refracted  towards  the  perpendicular,  and 
vice  versa.  If,  therefore,  an  object  be  examined  with  a  dry 
objective,  it  is  obvious  that  certain  rays  of  light  emerging  from 
the  denser  crown  cover-glass  into  the  rarer  medium,  air,  are 
refracted  so  far  from  the  perpendicular  as  to  fail  to  assist  in 
forming  the  image.  By  placing  a  medium  between  the  cover- 
glass  and  the  objective,  these  rays  are  utilized,  owing  to  the 
influence  of  the  dense  medium,  oil.     The  refractive  index  of  air 


60 


MODERN  MICROSCOPY 


A 


is  1*0,  that  of  water  1*33,  while  that  of  cedar- wood  oil  is  1*52. 

It  will  be  seen  from  this  that  the  utility  of  the  oil  must  be  very 

appreciable,  in  fact,  an  oil  immersion  lens  receiving  light  at  82° 

and  a  water  immersion  lens  receiving  light  at  96°  admit  the  same 

rays  as  a  dry  lens  of  180°,  and,  therefore,  divide  as  many  lines  to 

the  inch  as  the  maximum  number  possible 

with  a  dry  lens.    If  immersion  lenses  have 

greater  apertures  than  the  above-named, 

they  will  divide  finer  markings  than  any 

dry  lens,   and  they  can  be   theoretically 

carried  to  oil  and  water  angles  respectively 

of  180°. 

The  three  diagrams  (Fig.  22)  will  assist 

in     making    the    matter    clearer.      They 

represent  the  rays  of  light  passing  from 

an  object  mounted  in  optical  contact  with 

a  cover-glass,  or  in    a  sufficiently  dense 

medium  such  as  Canada  balsam,  through 

the  cover-glass. 

A  shows  the  front  lens  of  a  high-power 

Yia     22  Dry     Watep 

Immersion  and   Oil  dry  lens  over  the  object.     Only  the  least 

Immersion  Objectives,  inclined  of  the  three  pairs  of  rays  shown 

SHOWING    THE    KAYS    RE-  .  -i  i    • 

ceived  by  Each.  emanating  from  the  object  can  get  out  of 

the  cover-glass  into  the  air,  the  other  two 
being  so  oblique  that  they  suffer  total  reflection,  and  are  thus 
utterly  lost  to  the  dry  lens. 

B  represents  the  front  of  a  water  immersion  objective  over 
the  same  object.  Owing  to  the  refractive  index  of  water  being 
greater  than  that  of  air,  the  first  and  second  pair  of  rays  can 
now  pass  through  the  upper  surface  of  the  cover- glass  and  thus 
reach  the  front  lens,  with  a  corresponding  gain  in  brightness  of 
the  image  and  resolving  power.  The  most  oblique  pair  of  rays 
is,  however,  still  totally  reflected  and  lost  to  the  water  lens. 

C  shows  the  front  lens  of  an  oil  immersion  objective.  As 
the  refractive  index  of  the  immersion  oil  is  the  same  as  that  of 
the  cover-glass,  all  the  rays  now  travel  straight  on  from  object 
to  front  lens,  and  none  are  lost ;  hence  the  oil  lens  gives  the 
brightest  image  and  the  highest  resolving  power. 

There  is  another  feature  of  advantage  gained  by  the  use  of  an 


OPTICAL  CONSTRUCTION  61 

oil  immersion  lens.  The  refraction  caused  by  the  influence  of 
the  cover-glass  thickness  referred  to  on  p.  69  does  not  take 
place,  on  account  of  the  continuity  established  between  the 
objective  and  the  cover-glass  by  the  immersion  oil.  There  is, 
therefore,  no  necessity  for  such  objectives  to  be  provided  with  a 
correction  collar  for  variations  in  thickness  of  cover-glass ;  a 
slight  correction  of  the  same  kind  has,  however,  sometimes  to 
be  made  on  account  of  the  distance  which  the  object  may  be 
beyond  the  cover-glass  when  the  mounting  medium  has  not  the 
same  refractive  index  as  the  cover-glass.  This  can  be  efficiently 
effected  by  either  extending  or  shortening  the  body-length. 
Water  immersion  objectives  do  not  yield  so  high  an  aperture  as 
the  oil  immersions,  and  as  the  immersion  medium  is  not  of  the 
same  density  as  the  cover-glass  a  correction  collar  is  essential, 
but  there  are  subjects  with  which  oil  could  not  be  suitably  used, 
and  in  such  cases  the  water  immersion  lenses  have  to  be  chosen. 

Mono-bromide  of  naphthalin  is,  at  present,  only  used  with  one 
form  of  objective,  a  TV-inch,  by  Carl  Zeiss,  of  Jena,  having  a 
numerical  aperture  of  1*63.  The  refractive  index  of  this  medium 
is  1*657,  and  special  flint  cover-glasses  of  the  same  density  have 
to  be  employed  with  it.  This  restriction,  together  with  its  high 
price — £'40 — has  prevented  its  being  largely  used.  Those  who 
have  had  an  opportunity  of  working  with  one  have  spoken  in 
high  terms  of  the  beautiful  effects  it  yields. 

It  must  be  borne  in  mind  that  objectives  that  are  intended  to 
be  used  immersed  are  specially  corrected  for  the  specific  medium 
to  be  employed.  Ordinary  lenses  intended  for  use  dry  cannot  be 
advantageously  worked  immersed. 

An  important  point  in  working  with  immersion  objectives 
must  be  mentioned.  It  has  been  remarked  by  workers  that  the 
same  objective  does  not  always  give  equally  satisfactory  effects 
even  with  the  same  object ;  that  definition  has  gradually  im- 
proved after  a  short  time  in  use,  and  perfectly  good  objectives 
have  been  condemned  as  poor.  The  explanation  is  to  be  found 
in  the  difference  or  differences  of  temperature  of  the  object,  the 
immersion  oil,  and  the  objective.  Occasionally  the  cover-glass 
is  warm  after  a  specimen  has  been  hastily  prepared,  and  this 
might  produce  a  very  marked  effect.  The  only  way,  if  the 
parts  have  not  been  exposed  under  the  same  conditions,  is  to  set 


62  MODERN  MICROSCOPY 

up  the  microscope  and  let  it  stand  for  a  short  while,  so  that  all 
may  have  a  mean  temperature. 

A  note  may  be  added  on  the  removal  of  the  immersion  medium 
from  the  front  of  the  objective.  The  usual  method  is  to  wipe  it 
carefully  with  a  soft  handkerchief  of  silk  and  cotton  mixture  or 
a  clean  thin  chamois  leather.  This  is  usually  sufficient,  but  it 
must  be  done  immediately  after  use,  and  with  care.  Dr.  Henri 
van  Heurck  always  recommended  the  use  of  saliva  on  a  piece  of 
old  dry  linen  for  the  purpose,  and  stated  that,  '  thanks  to  the 
slight  quantity  of  soda  contained  in  the  saliva,  the  cleaning  is 
perfect  and  practically  instantaneous.' 

There  are  times  when,  by  oversight  or  force  of  circumstances, 
the  lens  is  left  uncleaned,  and  the  oil  drys  on  hard  and  is  difficult 
of  removal.  The  simplest  way  is,  then,  to  stand  the  objective 
on  its  screwed  end,  and  to  put  a  few  drops  of  the  immersion 
fluid  on  the  dry  deposit ;  this  will,  after  a  short  interval,  act  as 
a  solvent,  and  the  whole  will,  with  a  little  care  and  patience,  and 
perhaps  a  repetition  of  the  process,  be  easily  cleaned  off. 

APERTURES  OF  OBJECTIVES— ANGULAR  AND 

NUMERICAL. 

For  reasons  which  will  be  stated  hereafter,  it  will  be  seen  that 
on  the  aperture  possessed  by  an  objective  depends  the  fineness 
of  detail  that  it  is  capable  of  delineating — that  is,  the  number  of 
lines  per  inch  that  it  will  separate. 

Angular  Aperture. — Before  the  introduction  of  immersion 
objectives  the  ability  of  an  objective  to  resolve  fine  structure  was 
known  to  be  dependent  on  the  angle  formed  by  the  extreme  rays 
issuing  from  the  object  that  could  be  received  by  the  objective. 
This,  which  was  called  the  angular  aperture  of  a  lens,  is,  in  other 
words,  the  angle  of  the  cone  which  envelops  the  pencil  of  light 
that  is  received  by  the  objective  from  a  point  on  the  object. 

As  we  have  stated  in  the  description  of  immersion  lenses,  an 
oil  immersion  lens  receiving  light  at  82°,  and  a  water  immersion 
receiving  light  at  96°,  would  each  divide  as  many  lines  to  the 
inch  as  a  dry  lens  having  the  limiting  aperture  of  180°  (which 
in  practice  can  never  be  quite  reached),  and  as  the  immersion 
lenses  can  theoretically  be  carried  to  oil  and  water  angles  respec- 


OPTICAL  CONSTRUCTION  63 

tively  of  180°,  it  is  obvious  that  in  order  to  express  the  efficiency 
of  such  objectives  a  notation  must  be  employed  which  takes 
cognizance  of  the  medium  which  surrounds  the  front  of  the 
objective,  and  the  result  it  has  in  the  formation  of  the  image. 
This  is  achieved  by  means  of  the  system  termed  Numerical 
Aperture,  which  was  introduced  by  Professor  Abbe.  This  ex- 
presses the  efficiency  of  an  objective  to  allow  pencils  of  light  to 
pass  so  as  to  include  them  in  the  light  forming  the  image. 
Numerical  aperture  is  expressed  in  the  formula  n  sin  n.  n  signi- 
fies the  index  of  refraction  of  the  medium  by  which  the  objective 
front  is  enveloped,  and  u  equals  half  the  angle  of  aperture. 
Therefore,  by  multiplying  the  sine  of  the  semi-angle  of  aperture 
by  the  refractive  index  of  the  medium  in  which  that  angle  has 
been  measured,  the  numerical  aperture  (n  sin  u)  is  obtained. 

It  follows  from  this  that  the  greatest  value  which  the  nu- 
merical aperture  can  have  in  the  case  of  dry  lenses  is  unity, 
corresponding  to  an  angular  aperture  of  180°. 

If  we  are  aware  of  the  numerical  aperture  of  an  objective  we 
can  readily  ascertain  the  number  of  lines  per  inch  or  millimetre 
which  it  is  capable  of  dividing,  or,  in  other  words,  its  extreme 
power  of  resolution.  The  formula  is — twice  the  numerical 
aperture,  multiplied  by  the  wave-frequency*  of  the  light  used, 
equals  the  extreme  number  of  markings  per  inch  or  millimetre, 
according  as  the  calculation  may  be  made,  that  the  lens  will 
resolve,  t  Conversely,  if  the  extreme  limit  of  resolving  power 
be  known,  the  number  of  lines  per  inch  or  millimetre  that  it 
will  separate,  divided  by  the  wave-frequency  of  light  used,  equals 
twice  the  numerical  aperture. 

These  calculations  are  based  on  the  assumption  that  annular 
or  some  other  form  of  oblique  illumination  is  used.  With  a  solid 
cone  of  illumination  equal  to  the  numerical  aperture  of  the 
objective  no  fine  detail  is  visible  ;  it  becomes  blurred,  and,  in 
practice,  when  using  solid  cones  of  illumination,  it  is  usual  to 

*  The  wave-frequency  is  the  number  of  waves  contained  in  an  inch  or 
millimetre,  according  to  which  measure  is  used. 

t  The  mean  wave-length  of  white  light  is  0-5269  fi  (=48,200  to  an  inch). 
Taking  the  numerical  aperture  of  an  objective  as  TO  N.A.,  and  for  this 
purpose  doubling  it,  we  find  that  with  the  aperture  of  1*0  N.A.  96,400  lines 
(about)  per  inch  can  be  resolved  with  white  light  (48,200  x  double  the 
numerical  aperture  =  2,  produces  96,400). 


64  MODERN  MICROSCOPY 

make  them  fill  three-quarters  only  of  the  back  lens  of  the  objective. 
This  will  be  found  treated  on  p.  100  in  connection  with  condensers. 
Under  these  conditions  the  number  of  lines  per  inch  that  will 
be  resolved  by  the  objective  will  be  ascertained  by  multiplying 
the  wave-frequency  of  the  light  used  by  f ,  and  then  multiplying 
the  product  by  the  numerical  aperture.*  The  foregoing  is  probably 
the  estimate  from  practical  working,  but  the  theoretical  factor  is  £. 

Measuring  Numerical  Apertures. 

It  is  always  advantageous  for  the  worker  to  be  in  a  position  to 
measure  the  N.A.  of  objectives,  and  three  reliable  means  of  so 
doing  are  here  given. 

The  Apertometer. — To  enable  the  numerical  apertures  of 
objectives  to  be  taken  without  a  calculation,  Professor  Abbe 
devised  the  apertometer.  It  consists  of  an  almost  semicircular 
plate  of  glass,  having  the  diametrical  edge  ground  to  an  angle  of 
45°,  while  the  circumference  is  a  polished  cylindrical  surface. 
It  is  shown  in  Fig.  23. 

The  centre  of  the  semicircle  is  marked  by  a  silvered  disc,  a, 
having  a  very  small  central  aperture,  and  on  the  upper  surface 
on  the  periphery  it  is  provided  with  a  scale  of  divisions,  indicating 
both  angular  and  numerical  apertures.  The  manner  of  using  the 
apertometer  is  as  follows :  The  microscope  is  placed  in  a  vertical 
position,  and  the  apertometer  is  laid  on  the  stage,  with  the 
diametrical  edge  towards  the  limb  of  the  instrument.  The 
objective  that  it  is  desired  to  take  the  aperture  of  is  then  screwed 
on,  and  the  objective  focussed  on  the  plain  centre  of  the  silvered 
disc.  It  is  well  now  to  fix  the  apertometer  to  the  stage,  either 
by  springs  or  an  elastic  band,  to  prevent  its  moving.  The  two 
pointers,  b,  are  then  set  on  the  edge  of  the  circle  to  read  zero. 
The  draw-tube  and  eyepiece  with  which  the  silvered  disc  has  been 
set  are  removed,  and  at  the  lower  end  of  the  draw-tube  a  special 
objective  of  low  power,  that  is  supplied  with  the  apparatus,  is 
screwed  into  the  universal  thread,  which  should  be  there  fitted  in 
all  microscopes  of  high  class. 

The  cylindrical  edge  of  the  apertometer  is  then  illuminated  in 
front  and  at  the  sides  by  means  of    bull's-eye  condensers  and 

*  E.  M.  Nelson,  Jo urnal  of  the  Royal  Microscopical  Society,  1893,  p.  15 


OPTICAL  CONSTEUCTION 


65 


lamps,  or,  if  daylight  is  available,  it  will  be  easier  to  get  uniform 
brilliance  all  over  the  field  by  placing  the  microscope  on  a  table 
in  front  of  a  window,  and  using  bull's-eye  condensers  to  increase 
the  light. 

The  draw-tube  carrying  the  special  objective  and  the  eyepiece 
is  then  replaced  in  the  body  of  the  microscope,  and  the  image  of 
the  pointers  b  in  Fig.  23  is  brought  sharply  into  focus  in  the 
centre  of  the  field  by  slowly  extending  the  draw-tube,  being 
reflected  by  means  of  the  prismatic  diametrical  edge  of  the  glass 
plate.  While  looking  through  the  microscope  these  pointers  are 
then  each  moved  separately,  in  opposite  directions,  round  the 


0  2       0      0.2 


nD  =  1.615 

n  7~:~~  Abbe's  T 

C.Zeiss  Jena. 

Aoertometer. 


Fig.  23. — Abbe's  Apehtometek. 


outer  edge  of  the  apertometer,  until  they  are  set  exactly  on  the 
extreme  margins  of  the  back  lens.  The  reading  is  then  taken  by 
the  divisions  on  the  face  of  the  apertometer  against  which  the 
pointers  have  now  arrived.  In  the  case  of  an  oil  or  water  im- 
mersion lens,  the  medium  must,  of  course,  be  placed  in  front  of 
the  object-glass  during  the  examination. 

Caution  is  necessary  in  applying  this  instrument  to  some  of 
the  modern  objectives  with  unusually  large  lenses,  and  also  in 
applying  it  to  condensers.  The  reason  for  this  is  that  the 
auxiliary  objective  supplied  with  the  apertometer,  which  is  to 
be  screwed  to  the  end  of  the  draw-tube  in  order  to  produce  a 
microscope  for  clearly  observing  the  aperture  of  the  objective 
under  test,  is  of  too  small  a  clear  diameter  to  receive  all  the 
light  which  objectives  and  condensers  of  the  kind  mentioned  are 
capable  of  passing  ;  the  result  being  that  the  N.A.  obtained  under 


66 


MODERN  MICKOSCOPY 


Fig.  24. — Cheshire's 
Apertometer,  by 
R.  and  J.   Beck. 


such  conditions  is  too  small.  It  is  necessary  in  such  cases  to  do 
away  with  the  auxiliary  objective  altogether,  and  to  observe  the 
back  of  the  objective  under  test  by  looking  directly  at  it  down 
the  microscope-tube  without  the  intervention  of  any  lenses  what- 
ever. With  objectives  of  such  a  large  diameter  as  those  to  which 
this  warning  applies  there  is  no  difficulty  whatever  in  obtaining 
an  accurate  setting  by  naked-eye  observations. 

Cheshire's  Apertometer. — This  is  quite  a  simple  device,  and 
enables  a  sufficiently  exact  estimate  of  the  numerical  aperture  to 

be  obtained  for  most  purposes.  Familiarity 
with  it  renders  it  more  reliable  than  would 
be  expected  at  first  sight. 

The  instrument  as  made  by  R.  and  J.  Beck 
consists  of  a  circular  disc  of  glass  with  a 
mark  on  the  upper  surface  to  which  the 
object-glass  being  tested  is  focussed.  The 
lower  surface  is  ruled  with  a  series  of  con- 
centric rings,  each  of  which  is  placed  so  as 
to  correspond  to  0*1  N.A. 
The  method  of  use  is  to  place  the  apertometer  on  the  stage  of 
the  microscope,  focus  the  cross-lines  ruled  on  the  upper  surface, 
then  remove  the  eyepiece  and  count  the  number  of  rings  which 
show  through  the  back  lens  of  the  object-glass.  For  high  powers, 
when  the  lens  is  small,  and  consequently  the  rings  are  difficult  to 
count,  a  special  eyepiece  which  focusses  to  the  back  focal  plane 
of  the  object-glass  is  supplied,  which  is  inserted  in  place  of  the 
usual  eyepiece. 

Mr.  Conrady's  Method  for  Dry  Objectives. — In  the  case  of 
dry  objectives  the  numerical  aperture  may  be  determined  without 
expensive  apparatus  of  any  kind,  and  with  equal  accuracy  to 
that  obtainable  with  the  Zeiss  apertometer,  by  any  method 
suitable  for  determining  the  angle  of  the  objective. 
Perhaps  the  easiest  way  of  doing  so  is  the  following : 
On  a  dark  background,  such  as  a  table,  place  two  white  cards 
with  their  inner  edges  parallel  to  each  other  and  a  suitable 
distance  apart.  If  a  dry  objective  is  held  at  a  sufficient  distance 
above  the  table  and  directed  towards  a  point  midway  between 
the  two  cards,  an  image  of  the  latter  will  be  seen  at  the  back  of 
the  objective,  and  by  approaching  the  objective  to  the  table 


OPTICAL  CONSTEUCTION  67 

these  images  will  recede  from  one  another,  until  finally  they  can 
be  got  to  disappear  under  the  margin  of  the  back  lens. 

It  is  obvious  that  when  this  disappearance  takes  place  the 
inner  edges  of  the  two  cards  are  lying  in  the  direction  of  the 
most  oblique  rays  which  can  enter  the  objective.  In  other 
words,  they  form,  with  the  focal  point  of  the  objective,  its  angle 
of  aperture. 

In  order  to  get  numerical  results,  the  two  cards  must  be  placed 
at  a  measured  distance  apart,  and  the  objective  made  to  slide  up 
and  down  along  the  edge  of  a  divided  scale,  such  as  an  ordinary 
foot  rule. 

When  the  point  of  disappearance  of  the  images  of  the  inner 
edges  of  the  cards  has  been  reached,  the  distance  from  the  front 
of  the  objective  to  the  table  is  read  off  on  the  scale.  This  is 
further  diminished  by  the  working  distance  of  the  objective, 
which  must  be  separately  measured,  and  the  numerical  aperture 
is  then  obtained  as  follows  : 

1.  If  a  table  of  trigonometrical  functions  is  available,  divide 
the  reduced  distance  from  objective  to  table  by  half  the  distance 
between  the  inner  edges  of  the  two  white  cards.  The  quotient 
is  the  co-tangent  of  half  the  angle  of  aperture  ;  find  its  value  in 
the  table,  and  take  from  the  table  the  sine  of  the  same  angle. 
This  is  the  desired  numerical  aperture. 

2.  If  no  trigonometrical  table  is  available,  the  numerical  aper- 
ture is  found  as  follows  : 

Square  the  reduced  distance  from  objective  to  table,  also  half 
the  distance  between  the  two  cards.  Add  the  two  squares  together 
and  extract  the  square  root  of  the  result.  Then  the  numerical 
aperture  is  found  as  the  quotient  of  half  the  distance  between  the 
cardboards,  and  the  value  of  the  above  square  root. 

In  order  to  make  the  calculation  involved  as  simple  as  possible, 
it  is  manifestly  an  advantage  to  make  one  of  the  lengths  entering 
into  the  calculation  unity,  which  is  easily  done  by  placing  the 
two  cards  a  distance  of  two  units  apart. 

For  objectives  of  low  numerical  aperture  2  inches  will  be  found 
a  suitable  distance;  for  those  of  higher  numerical  aperture  a 
distance  of  2  decimetres.  This  last  measurement  is  suggested  so 
that  the  experiment  may  be  in  its  simplest  form. 

Numerical  Aperture  and  Power. — As  we  have  before  remarked 


68  MODERN  MICROSCOPY 

magnifying  power  is  not  the  only  quality  necessary  for  the  ob- 
servation of  minute  structure.  The  power  to  delineate  fine  detail 
is  still  more  dependent  on  the  numerical  aperture  of  the  objective. 
It  has  been  explained  by  Dr.  Dallinger  in  '  Carpenter  on  the 
Microscope,'  and  will  be  evident  from  a  consideration  of  the 
preceding  remarks  concerning  numerical  aperture,  that  two 
objectives — one  of  much  greater  magnifying  power  than  the 
other,  but  both  having  only  the  same  numerical  aperture — 
will  only  divide  the  same  amount  of  detail,  the  higher  power 
exhibiting  it  on  a  larger  scale.  That  is,  supposing  with  a  J-inch 
objective  of  0'90  N.A.  certain  structure  were  presented,  and  then 
a  ^-inch  objective  with  just  double  the  magnification,  but  with 
the  same  N.A.,  were  afterwards  used,  there  would  be  no  further 
power  of  resolution  in  the  J  than  in  the  \.  It  might  be  possible 
to  make  an  objective  of  very  low  power  of  sufficiently  high 
aperture  to  divide  very  minute  details,  but  this  would  be  useless 
unless  the  objective  would  bear  a  sufficiently  deep  eyepiece  to 
enable  the  human  eve  to  see  it.  It  therefore  becomes  neces- 
sary  that  a  ratio  of  aperture  to  power  should  be  established. 
Mr.  Nelson  has  suggested  that  a  standard,  termed  the  '  optical 
index'  (O.I.),  should  be  adopted  for  this  purpose,  to  indicate  the 
numerical  aperture  that  should  be  given  to  an  objective,  if  it  be 
intended  that  the  eye  should  see  in  the  image  as  fine  detail  as  it 
could  divide  in  a  real  object  of  the  same  size.  It  is  ascertained 
by  multiplying  the  numerical  aperture  of  an  objective  by  1,000, 
and  dividing  by  the  initial  magnifying  power  of  the  objective.* 
If  a  microscope  is  required  to  show  all  that  keen  eyes  are  able 
to  appreciate,  then  0*26  N.A.  must  be  given  to  it  for  every  100 
diameters  of  magnification.  If  we  limit  the  power  of  the  eye- 
piece of  such  a  microscope  to  10,  then  the  objective  must  have 
0*26  N.A.  for  each  10  diameters  of  initial  magnifying  power. 
The  optical  index,  therefore,  of  an  objective  which,  with  an  eye- 
piece magnifying  10  diameters,  will  yield  all  that  it  is  possible 
for  a  normal  eye  to  appreciate,  will  be  26'0.  In  practice  it  is 
found  possible  to  employ  eyepieces  giving  higher  magnifications 
than  those  mentioned  in  Mr.  Nelson's  rule,  and  these  would,  of 
course,  enlarge  the  image. 

*  E.  M.  Nelson,  Journal  of  the  Royal  Microscojrical  Society,  February, 

i8yo. 


OPTICAL  CONSTKUCTION  69 

Although  large  apertures  are  the  pride  of  those  whose  ultimate 
ambition  in  matters  microscopical  seems  to  be  bounded  by  the 
endeavour  to  resolve  the  markings  upon  diatomaceous  frustules, 
t  is  doubtful  whether  for  the  ordinary  amateur  there  is  a  real 
necessity  for  the  extremely  large  apertures.  Lenses  having  such, 
require  great  skill  and  care  in  manufacturing  and  adjusting,  and 
are  consequently  expensive  ;  and  if  the  ordinary  work  of  an 
amateur  is  proposed  to  be  conducted,  and  not  original  scientific 
research,  objectives  of  medium  aperture  will  usually  be  found  to 
meet  his  requirements  thoroughly. 

THE  INFLUENCE  OF  THE  COVER-GLASS. 

As  a  rule,  objectives  are  corrected  for  a  specified  thickness  of 
cover-glass,  which  is  placed  over  the  object  to  protect  it.  These 
cover-glasses,  however,  vary  considerably  in  thickness,  and  con- 
sequently by  refraction  disturb  the  corrections  of  the  objectives. 
An  objective  which  gives  crisp  definition  when  an  object  that 
has  no  cover-glass  to  it  is  being  viewed,  will  not  define  so  clearly 
if  a  thin  one  be  applied,  and  the  greater  the  thickness  of  the 
cover-glass  the  more  will  the  image  be  deteriorated. 

In  other  words,  spherical  aberration  in  the  sense  of  under- 
correction  is  introduced  when  the  cover-glass  is  thinner,  and  in 
the  sense  of  over-correction  when  the  cover-glass  is  thicker,  than 
that  for  which  the  lens  was  adjusted.  This  spherical  aberration 
arises  from  the  refraction  of  the  rays  in  the  plane  surfaces  of  the 
cover-glass  and  objective  front  respectively ;  it  is  negative  for  the 
cover-glass  surface,  and  positive  in  the  front  lens  piano,  the  latter 
preponderating  owing  to  the  greater  diameter  of  the  cone  of  rays 
from  the  object  where  it  enters  the  front  lens.  With  the  correct 
thickness  of  cover-glass,  the  remaining  under-correction  is  exactly 
balanced  by  an  equal  over-correction  in  the  lenses  of  the  objective, 
but  a  thin  cover-glass  produces  insufficient  over-correction,  the 
diameter  of  the  cone  of  rays  being  too  small  when  it  meets  the 
surface  of  the  cover-glass.  A  thick  cover-glass  produces  the 
opposite  effect — that  is,  the  cone  of  rays  is  too  large. 

Low  powers  are  not  so  sensitive  to  this  influence  as  high  ones. 
There  are  two  means  of  correcting  this.  Dry  objectives  having 
a   large   numerical   aperture  are  often   provided   with  what   is 


70 


MODERN  MICROSCOPY 


termed  '  a  correction  collar,'  whereby  the  two  back  combinations 
of  the  objective  are  removed  farther  from,  or  brought  closer  to, 
the  front  lens  or  lenses.  Fig.  25  shows  the  manner  in  which 
this  is  effected  in  one  of  Zeiss's  lenses  :  bb  is  the  correction 
ring,  by  turning  which  the  distance  between  the  upper  lenses 
and  the  two  lower  lenses  is  varied.  With  such  a  correction 
collar  a  worker  is  undisturbed  by  thickness  of  cover-glass, 
because  he  has  within  certain  limits  the  means  at  his  disposal 
in  the  objective  itself  for  correcting  same.  The  use  of  this  cor- 
rection collar  almost  requires  a  personal  demonstration,  and  to 

set  it  at  the  exact  point  that  yields  the 
best  result  is  a  matter  of  extreme  delicacy, 
which  can  only  be  accurately  done  as  the 
result  of  experience  and  with  the  aid  of 
a  critical  eye.  It  is  hardly  to  be  recom- 
mended to  students,  because  they  will 
not  usually  afford  the  time  and  trouble 
necessary  to  get  such  perfect  results ; 
consequently  there  is  a  growing  tendency, 
except  in  the  apochromatic  objectives,  to 
have  the  lenses  mounted  in  a  rigid  setting, 
corrected  for  a  specific  tube-length  and 
thickness  of  cover-glass.  With  the  fixed 
setting,  if  a  different  thickness  of  cover-glass  be  used  than  that 
for  which  the  objective  was  designed,  correction  can  be  made  by 
altering  the  tube-length  of  the  microscope.  This  has  the  same 
effect  as  altering  the  distance  between  the  lenses.  Supposing  we 
had  an  objective  adjusted  for  a  6-inch  tube,  with  a  'B'  eyepiece, 
on  a  cover-glass  0*008  inch  thick  (this  is  about  the  average  thick- 
ness adopted  by  opticians),  and  we  wished  to  examine  an  object 
having  another  thickness  of  cover — say  0'005 — we  should  at  once 
notice  that  the  performance  was  not  so  good,  and  in  order  to  im- 
prove it  we  should  have  to  make  the  body  longer.  This  difference 
of  cover-glass  thickness,  with  a  good  J-inch  objective,  would 
necessitate  a  10-inch  body.  On  the  other  hand,  if  the  cover  were 
0"01  inch  thick,  the  length  of  the  body  would  have  to  be  diminished 
below  that  for  which  the  object  had  been  designed,  to  obtain  the 
best  results — that  is,  for  a  thin  cover  the  body-tube  would  have  to 
be  lengthened,  while  for  a  thick  one  it  would  have  to  be  shortened, 


Fig.  25. — Correction 
Collar  (Zeiss). 


OPTICAL  CONSTRUCTION  71 

and  the  finer  the  quality  of  the  objective  the  more  sensitive 
would  it  be  to  cover-glass  thickness.  This  system  of  correcting 
by  draw-tube,  however,  has  one  drawback,  and  that  is,  that  the 
power  is  varied  in  correcting,  and,  of  course,  the  focus  is  altered. 
From  the  considerations  here  named,  it  will  be  found  advan- 
tageous if  the  microscope  be  provided  with  a  means  of  lengthen- 
ing the  body  by  draw-tubes  to  12  inches,  and  on  the  other  hand, 
when  the  draw- tubes  are  closed,  of  having  the  body  shorter  than 
the  Continental  length  (6  inches).  In  order  that  the  best  adjust- 
ment may  be  made,  it  is  essential  that  one  of  the  draw-tubes  be 
actuated  by  rack  and  pinion,  and  the  convenience  of  this  arrange- 
ment cannot  be  too  strongly  urged  upon  microscopists.  Messrs. 
Baker,  Beck,  and  Watson  and  Sons  have  adopted  it  in  their 
large  models,  and  it  has  evidently  met  with  considerable  appre- 
ciation. 


DIRECTIONS  FOR  USING  A  CORRECTION  COLLAR 
AND  CORRECTING  BY  TUBE-LENGTH. 

This  may  be  accomplished  in  a  systematic  manner  if  it  be 
borne  in  mind  that  the  aim  is  to  eliminate  spherical  aberration, 
which  defect  may  be  defined  as  a  difference  of  focus  between  the 
central  and  marginal  zones  of  an  objective.  Hence  the  correct 
tube-length  or  the  best  position  of  the  correction  collar  has  been 
found  when  some  strongly  marked  detail  or  outline  of  the  object 
remains  in  exact  focus  under  any  change  of  illumination,  say 
from  a  small  to  a  large  diaphragm  opening  beneath  the  con- 
denser, or,  better  still,  by  changing  the  illumination  from  central 
to  very  oblique,  these  changes  being  made  with  great  care,  so  as 
not  to  disturb  the  other  adjustments. 

The  following  process  will  be  the  safest  and  quickest :  Start 
with  the  shortest  tube-length,  or  when  there  is  a  correction  collar, 
with  the  position  corresponding  to  the  thickest  cover-glass  ;  care- 
fully focus  some  sharp  outline  with,  say,  a  \  central  cone,  then 
change  to  a  J  cone,  or,  better  still,  to  very  oblique  light.  Unless 
the  object — owing  to  an  exceptionally  thick  cover-glass,  or  a 
very  badly  adjusted  lens — is  beyond  the  range  of  your  adjust- 
ments, you  will  find  evidence  of  under-correction — that  is,  the 


72  MODERN  MICROSCOPY 

lens  will  have  to  be  brought  closer  to  the  object  with  the  wide 
cone,  or  oblique  light,  than  with  central  light. 

Gradually  lengthen  the  tube,  or  turn  the  collar,  repeating  the 
above  observation  after  each  change,  until  all  evidence  of  spherical 
aberration  has  disappeared  ;  the  instrument  is  then  in  correct 
adjustment  within  your  own  limits  of  vision. 

It  is  advisable  to  start  with  the  adjustment  corresponding  to 
the  thickest  cover,  for  the  simple  reason  that  this  lessens  the 
danger  of  running  through  the  cover-glass  and  destroying  the 
object,  and  possibly  the  front  lens  of  the  objective,  when  dealing 
with  a  lens  of  a  short  working  distance. 

The  difference  between  an  objective  adapted  to  a  6-inch  and 
that  for  a  10-inch  tube  is,  that  in  the  latter  case  the  back  com- 
binations of  the  objective  are  brought  closer  to  the  front  lenses. 
This  gives  a  slightly  increased  aperture.  The  majority  of  cover- 
glasses  that  are  purchased  and  a  large  number  of  those  used 
over  commercial  objects  are  more  than  0007  inch  thick  ;  0*007 
inch  is  a  medium  thickness  of  cover-glass,  but  the  tendency  is  to 
use  thicker  ones.  It  will  be  found  a  great  advantage  to  buy  only 
such  objectives  as  are  corrected  for  a  medium  tube-length,*  and 
having  the  rackwork  before  referred  to  fitted  to  the  microscope- 
tube,  sufficient  latitude  would  still  be  allowed  if  a  thinner  cover- 
glass  were  met  with ;  but  it  would  often  be  found  necessary  to  close 
the  draw-tubes  down  to  6  or  7  inches,  in  order  to  get  the  best 
correction  for  the  thick  cover-glasses  that  are  commonly  used. 

We  may  here  clear  up  another  question  that  occasionally 
arises.  If  a  J-inch  objective  is  corrected  for  a  6-inch  tube- 
length,  it  does  not  give  a  magnification  of  60  diameters  at  6 
inches.  The  powers  of  all  objectives  are  calculated  for  a  10-inch 
tube-length,  therefore  the  full  total  benefit  is  not  obtained  from 
an  objective  when  used  at  6  inches,  but  only  six- tenths  of  it. 
Of  course,  with  the  lessened  magnification  at  6  inches  a  brighter 
field  is  produced,  and  a  deeper  power  of  eyepiece  is  found  per- 
missible. This  is  rather  an  important  item  in  testing  an  ob- 
jective, because  an  objective  at  10  inches  would  be  yielding 
about  two-thirds  more  magnification  than  at  6  inches,  and  its 

*  Mr.  Conrady  has  advocated  that  all  objectives  should  be  corrected  for  a 
tube -length  of  8  inches,  and  with  excellent  reason,  for  such  an  arrangement 
would  be  a  practical  step  in  the  solution  of  a  difficult  problem. 


OPTICAL  CONSTRUCTION  73 

powers  would  be  much  more  severely  tested  than  if  employed  at 
6  inches. 

It  would  be  a  great  advantage  to  the  microscopists  if  opticians 
would  mark  exactly  the  focal  power  and  precise  numerical  aper- 
ture of  their  objectives  upon  them.  In  order  that  objectives 
may  appear  to  have  a  large  ratio  of  aperture  to  power,  they  are 
often  put  forward  as  possessing  a  considerably  lower  power  than 
they  actually  have.  For  instance,  a  so-called  1-inch  often  turns 
out  to  be  nearer  §-inch,  J-inch  about  TVmch,  A-inch  to  have  the 
power  of  J-inch,  and  yV-inch  in  some  instances  to  be  ^-inch. 
It  has  become  such  an  acknowledged  fact  that  the  act  of  mis- 
representation involved  seems  to  be  condoned.  This  is  a  state 
of  things  which  should  not  be.  Opticians  must  be  aware  of  the 
misdescription  and  the  immensity  of  trouble  that  is  caused  by  it. 
We  must,  therefore,  advise  microscopists  not  to  rely  on  the 
powers  marked  on  their  objectives,  but  to  ascertain  them  for 
themselves,  and  the  best  way  to  do  it  is  to  project  the  image  of 
a  micrometer,  without  any  eyepiece  in  the  body-tube,  on  a  screen 
20  inches  distant  from  the  back  lens  of  the  objective.  Measure 
with  a  foot  rule  the  distance  apart  of  the  lines  so  projected,  and 
supposing  that  each  hundredth  of  an  inch  measured  on  the  screen 
1  inch,  that  would  represent  a  magnification  of  100  diameters  ; 
divide  the  distance  used  (20  inches)  by  the  magnification  found 
(100  diameters),  and  the  result  (T2o°o  or  A-inch)  is  the  equivalent 
focus  or  '  power '  of  the  objective. 

TESTING  OBJECTIVES. 

It  is  a  somewhat  difficult  matter  for  the  novice  to  decide  for 
himself  as  to  the  quality  of  object-glasses.  Such  work  needs 
experience,  judgment,  and  a  trained  eye.  The  writer  has  met 
with  people  who  have  not  been  able  to  distinguish  the  difference 
in  performance  between  an  uncorrected  single  French  lens  and  a 
first-class  achromatic.  This,  of  course,  was  due  entirely  to  a 
lack  of  that  perception  of  microscopical  detail  which  can  only 
be  acquired  by  intimacy  with  objectives  and  their  qualities. 
Especially  is  this  true  in  lenses  of  the  highest  grade.  We 
propose,  therefore,  to  give  a  few  hints  which,  if  not  of  so  much 
use  in  the  initial  stage,  may  be  of  aid  at  a  later  period. 


74  MODEEN  MICROSCOPY 

Flatness  of  Field. — This  feature,  however  much  it  may  be 
appreciated,  and  greatly  as  it  is  to  be  desired,  is,  unfortunately, 
impossible  of  association  with  objectives  of  fine  quality.  With 
low  powers  up  to  J  inch  it  is  generally  obtainable  for  a  consider- 
able portion  of  the  field,  especially  with  objectives  of  small 
numerical  aperture.  The  compromise  which  is  to  be  made  to 
secure  it  is  not  such  in  the  low  powers  as  to  materially  affect 
the  general  performance.  It  cannot,  however,  be  given  in  objec- 
tives of  medium  and  high  power.  A  well-corrected  objective 
inevitably  has  a  curved  field,  and  the  more  perfectly  it  is 
corrected,  the  more  apparent  does  it  become.  This  has  become 
increasingly  recognized,  and  it  is  now  conceded  that  it  is  better 
to  get  the  utmost  perfection  of  definition  in  the  central  zone 
rather  than  that  sharpness  should  be  sacrificed  to  flatness  of 
field.  Flatness  of  field  in  any  other  than  low-power  objectives 
cannot  therefore  be  expected  except  at  the  expense  of  inferior 
definition,  and  this  to  the  critical  worker  would  be  intolerable. 

It  must  be  noted,  however,  that  every  part  of  the  field  can  be 
separately  brought  into  view  with  all  well-corrected  lenses  by 
slightly  altering  the  focus  with  the  fine  adjustment. 

Colour. — Dr.  Carpenter's  old  test  for  achromatism — the  ex- 
amination of  the  cells  in  a  thin  section  of  deal — will  give  a  very 
good  idea  of  the  colour  corrections  of  objectives.  For  high 
powers,  the  markings  on  a  frustule  of  the  diatom  Pleurosigma  for- 
mosinn  are  an  excellent  test.  With  the  apochromatic  objectives 
these  come  out  quite  black  and  white,  while  with  those  of  the 
achromatic  series  any  outstanding  colour  is  at  once  revealed. 
Another  method  is  the  mercury  test  adopted  by  opticians.  A 
small  globule  of  mercury  is  placed  on  a  slip  of  ebonite,  and  a 
piece  of  whalebone  or  watch-spring  is  made  to  snap  on  it,  causing 
the  globule  to  split  up  into  numerous  particles  of  exceedingly 
minute  size.  These  globules  are  then  examined  with  the  objec- 
tive, and  can  be  illuminated  by  means  of  a  bare  gas-jet,  lamp, 
or  daylight.  Outstanding  colour  will  be  revealed  by  the 
globules.  A  cover-glass  of  proper  thickness  must  be  interposed 
when  submitting  lenses  of  considerable  numerical  aperture  to 
these  tests. 


OPTICAL  CONSTRUCTION 


75 


The  Abbe  Test- Plate. 

The  most  satisfactory  way  of  testing  an  objective  that  is  at  the 
disposal  of  him  who  would  learn  the  whole  inwardness  of  his  lens 
is  the  Abbe  test-plate.  A  considerable  amount  of  experience  will 
be  required  to  use  it  advantageously,  but  it  discloses  at  once, 
when  employed  by  one  who  has  learnt  to  appreciate  its  signifi- 
cance, any  mechanical  inaccuracies  that  may  exist.  Directions 
for  using  accompany  each  plate,  but  the  worker  will  quickly 
mark  out  for  himself  a  line  which  experience  will  show  him  is 
the  best  for  ascertaining  whether  the  lens  is  accurately  centred 
and  the  state  of  the  corrections  for  spherical  and  chromatic 
aberrations.     The  test-plate  itself  consists  of  six  discs  of  cover- 


Fig.  26. — Abbe  Test-Plate. 


glass,  all  of  different  specified  thicknesses,  and  embracing  such 
a  range  as  objectives  are  likely  to  be  corrected  for. 

On  the  under  surface,  lines  are  ruled  in  a  deposit  of  silver,  and 
the  covers  are  mounted  on  an  ordinary  3x1  slip.  The  ruled 
lines  are  coarse,  and  can  be  separated  with  a  low-power  objective. 
The  procedure  adopted  by  the  writer  is  as  follows : 

The  tube-length  should  be  that  for  which  the  objective  is 
ostensibly  corrected.  An  eyepiece  of  high  power  and  a  sub-stage 
condenser,  giving  a  solid  cone  equal  to  at  least  two-thirds  the 
total  aperture  of  the  objective,  are  used. 

In  this  connection  it  may  be  mentioned  that  it  will  be  found 
advantageous  to  have  an  e}7epiece  which  permits  of  some  degree 
of  over-  or  under-correction  being  obtained — such,  for  instance, 
as  will  be  afforded  by  the  Holoscopic  eyepiece  described  on 
p.  85.  Those  who  have  not  such  a  convenience  may  unscrew  the 
eye  lens  of  the  Huyghenian  eyepiece  and  so  secure  some  slight 
modification  of   correction.     The  object  of    this  is  to  ascertain 


76  MODERN  MICROSCOPY 

whether  the  lines  may  be  rendered  free  from  coloured  edges  or 
with  the  same  colour  on  both  edges  of  all  the  lines  in  the  field. 
If  in  this  preliminary  step  it  be  found  that  the  definition  is 
unsatisfactory,  thicker  and  thinner  cover-glasses  should  be  tried  ; 
and  in  the  event  of  failure  to  secure  good  definition  in  this  way, 
and  no  reasonable  alteration  of  tube-length  will  produce  the 
desired  effect,  the  objective  may  be  safely  rejected  as  bad. 

It  probably  will  be  found  that  under  one  of  the  cover-glasses 
the  lines  will  appear  satisfactorily  defined,  in  which  case  the 
centring  may  be  examined. 

Defective  Centring  shows  itself  (a)  by  the  impossibility  of 
removing  the  coloured  edges  of  the  lines  all  over  the  field  even 
when  the  eyepiece  is  adjusted  as  described  above,  the  edge 
colouring  being  more  apparent  on  one  side  of  the  field  than  the 
other ;  (b)  by  unequal  definition  of  the  two  edges  of  the  central 
lines,  one  edge  appearing  sharp  or  nearly  so,  while  the  other 
edge  is  seen  double  or  foggy. 

Spherical  Aberration. — The  fact  that  the  objective  will  bear 
high-power  eyepieces  on  the  test-plate  in  a  satisfactory  manner 
is  in  itself  proof  of  good  correction  in  this  respect,  but  the 
following  is  a  further  excellent  and  convincing  test : 

Place  a  diaphragm  beneath  the  condenser  having  an  aperture 
that  will  cause  the  condenser  to  yield  a  cone  of  illumination 
equal  to  one-fourth  the  N.A.  of  the  objective  under  examination, 
and  while  observing  the  lines  change  the  position  of  this  dia- 
phragm from  central  to  extremely  oblique,  the  obliquity  being 
in  a  plane  at  right  angles  to  the  direction  of  the  lines.  This  is 
best  performed  by  means  of  one  of  the  mechanical  condenser 
carriers,  such  as  are  provided  in  the  Continental  microscoj^es. 
If  the  lines  remain  sharp  throughout,  the  corrections  for  spherical 
aberration  are  eminently  satisfactory ;  but  should  a  difference  of 
focus  occur,  to  avoid  all  chances  of  erroneous  deduction  the  other 
discs  should  be  examined  to  insure  that  the  proper  thickness  of 
cover  is  being  used ;  also  the  tube-length  might  be  varied, 
and  if  after  these  precautions  it  is  still  found  that  there  is  a 
difference  of  focus  over  the  intermediate  position  of  the  diaphragm, 
the  existence  of  a  spherical  zone  is  at  once  demonstrated.  This 
process  enables  the  best  tube-length  and  thickness  of  cover  for 
the  objective  to  be  discovered  with  accuracy. 


OPTICAL  CONSTEUCTION  77 

Chromatic  Correction. — Tests  for  this  should  be  made  in  the 
same  manner  as  for  the  spherical  correction.  Under  the  same 
conditions  an  apochromatic  should  show  practically  no  colour, 
or,  at  the  most,  barely  distinguishable  traces  of  tertiary  tints. 
Semi-apochromatics,  or  lenses  of  fine  correction,  will  show 
narrow  bands  of  pale  green  (apple-green)  on  one  side,  and  faint 
purple  (or  claret)  on  the  other  side,  of  each  line,  and  the  same 
colours  or  tints  should  appear  whether  the  diaphragm  be  used 
centrally  or  obliquely,  the  width  of  the  colour  bands  only 
changing;  further,  good  definition  should  be  yielded  under  all 
circumstances.  Ordinary  lenses  will  generally  show  the  best 
colour  correction  for  some  intermediate  zone.  If  they  exhibit 
broad  bands  of  primary  colour — yellow  or  blue — with  very 
oblique  light,  the  definition  will  be  found  to  be  bad. 

Curvature  of  Field. — There  is  one  point  concerning  which 
some  slight  difficulty  may  arise  in  connection  with  the  curvature 
of  the  field.  Absence  of  flatness  of  field,  which  is  inherent  in 
the  construction  of  all  latter-day  objectives,  and  particularly  so 
in  those  of  high  power  and  large  aperture,  is  not  regarded  as  a 
fault,  but  coma  in  an  objective  at  first  sight  gives  the  same 
appearance ;  the  difference,  however,  is  this :  when  the  latter 
fault  exists  only  the  central  line  or  those  nearest  to  it  can  be 
focussed  sharply,  those  towards  the  margin  remaining  indistinct 
or  ill-defined  when  refocussing  is  attempted  ;  in  a  well-corrected 
lens  all  of  the  lines  will  become  sharp  and  distinct  when  the 
particular  zone  is  adjusted  for. 

It  will  be  found  advantageous  to  confirm  the  observations  on 
the  test-plate  by  examinations  of  known  test  objects,  and  with 
practice  the  two  together  will  soon  enable  reliable  estimates  to 
be  formed  of  the  quality  of  objectives. 

Tests  for  Definition. — Use  an  eyepiece  with  the  objective 
under  examination  that  will  give  a  total  magnification  in 
diameters  equal  to  one  thousand  times  the  numerical  aperture 
of  the  objective — that  is,  if  a  J  -inch  objective,  having  a  magni- 
fying power  of  20  diameters  on  the  10-inch  tube,  had  a  numerical 
aperture  of  0*45,  an  eyepiece,  the  magnification  of  which  was  22| 
diameters,  would  be  necessary  to  give  the  required  450  diameters. 
If  an  objective  bears  this  without  serious  breaking  down  its 
definition  may  be  considered  to  be  good.     This  test  has  the 


78  MODERN  MICROSCOPY 

advantage  of  being  based  on  a  rational  foundation,  the  ratio 
being  the  same  as  an  eyepiece  power  of  50  to  each  inch  of 
aperture  in  an  ordinary  telescope.  This,  again,  is  equal  to 
what  would  be  seen  of  an  object  if  looked  at  through  a  pin-hole 
-^o  inch  in  diameter,  beyond  which  the  outlines  of  objects  fail  in 
clearness. 

For  objectives  varying  in  power  from  2  inches  to  ^  inch, 
nothing  is  better  as  a  test  than  the  proboscis  of  a  blowfly.  The 
spines  in  the  central  portion  of  the  tongue  should  each  show 
a  well-defined  point.  For  high -power  objectives  the  internal 
markings  of  Triceratum  and  Pleiirosigma  angulatum,  also  the 
markings  on  the  scales  of  Podura  (Lepidocyrtus  curvicoUis),  are 
the  most  suitable. 


CHOICE  OF  OBJECTIVES. 

The  best  objectives  for  a  novice  at  starting  would  be  2-inch, 
1-inch,  and  i-inch.  The  2-inch  will  be  found  extremely  useful 
for  large  specimens,  while  the  1-inch,  which  is  considered  the 
working-glass  of  the  average  microscopist,  will  with  a  higher 
power — namely,  the  J-inch — show  him  some  of  the  minuter 
detail  which  sooner  or  later  he  will  wish  to  make  himself 
acquainted  with.  If  more  object-glasses  than  these  be  required, 
we  should  recommend  the  J-inch  as  an  intermediate  between 
the  1-inch  and  the  J-inch,  and  for  a  higher  power  a  TV-inch  oil 
immersion  objective  should  be  added.  For  medical  work  the 
|-inch  is  invariably  chosen  with  the  J-inch.  It  is  well  to  buy 
only  such  low-power  objectives  as  have  double  combinations. 
Some  of  the  cheaper  ones  consist  of  two  or  three  lenses  balsamed 
together  in  one  combination  only ;  with  these  there  is  an  in- 
sufficiency of  aperture,  and  good  definition  and  flatness  of  field 
cannot  be  obtained.  All  the  best  low-power  lenses  are  con- 
structed with  two  pairs  or  more  of  lenses  set  a  little  distance 
apart,  and  can  be  readily  recognized.  Of  the  apochromatic  series 
of  Zeiss  our  choice  would  be  the  24,  12,  6,  and  2  millimetre 
objectives  if  for  the  English  tube-length,  or  the  16,  4,  and  2 
if  for  the  Continental  tube-length.  Whether  the  2 -millimetre 
objective  should  be  that  with  the  maximum  numerical  aperture  of 
1*40  or  that  having  1*30  depends  on  the  work  to  be  done,  and  the 


OPTICAL  CONSTRUCTION  79 

worker.  The  lens  of  lower  aperture  is  less  liable  to  injury  than 
the  other,  and  is  consequently  much  more  largely  used.  Generally 
speaking,  1*30  will  be  found  sufficient,  but  that  of  1*40  will  be 
the  lens  for  the  worker  who  requires  the  utmost  capacity  in  his 
lens,  and  will  give  it  due  care.  A  J-inch  with  the  remarkable 
working  distance  of  1  millimetre  has  been  introduced  by  Watson 
and  Sons,  and  is  excellent  in  performance,  being  of  the  semi- 
apochromatic  variety.  Swift  and  Sons'  pan-aplanatic  lenses  are 
deservedly  popular,  while  the  objectives  of  Leitz  and  Zeiss  are 
uniformly  good.  As  previously  remarked,  competition  has 
caused  all  the  makers  to  bring  their  objectives  to  a  high  level 
of  perfection,  and  the  novice  will  be  quite  safe  in  equipping  him- 
self with  those  of  any  of  the  leading  English  opticians.  The  pro- 
ductions of  American  opticians  are  but  little  known  in  England, 
yet  they  are  said  in  many  instances  by  competent  judges  to  be 
of  exceptional  quality,  Messrs.  Bausch  and  Lomb  and  the  Spencer 
Lens  Company  having  high  reputation  for  their  lenses. 

Of  course,  different  requirements  would  necessitate  the  selection 
of  special  objectives,  but  a  practical  microscopist  or  any  micro- 
scope manufacturer  would  be  able  to  advise  on  the  matter. 


HOW  A  MICROSCOPE  OBJECTIVE  IS  MADE. 

The  knowledge  of  the  processes  involved  in  the  manufacture 
of  micro-objectives  is  so  vague  that  a  brief  description  will  be  of 
interest  to  most  readers.* 

The  mathematician  so  computes  an  objective  that,  when  it 
is  constructed  from  the  optical  glasses  which  he  has  chosen  as 
suitable  for  their  refractive  and  dispersive  qualities,  with  the 
component  lenses  shaped  to  the  thickness  and  curvature  he  has 
prescribed,  and  finally  mounted  and  accurately  centred  at  the 
computed  distances,  it  will  exactly  realize  his  intentions. 

In  optical  glass  he  has  a  wide  choice,  but  when  a  melting  is 
exhausted  it  rarely  happens  that  an  exact  duplicate  can  be 
obtained  from  the  glass  manufacturers,  two  meltings  seldom 
agreeing  with  sufficient  precision  in  their  refractive  and  dispersive 

*  This  description  was  given  at  a  demonstration  given  at  the  Quekett  Club 
by  Mr.  F.  W.  Watson  Baker,  F.R.M.S. 


80  MODERN  MICROSCOPY 

powers.  It  therefore  becomes  necessary  to  re-compute  the  lens 
for  each  change  of  its  constituents. 

Incidentally,  there  is  here  an  explanation  of  the  reason  why 
the  replacing  of  any  portion  of  an  objective  system  years  after 
its  manufacture  rarely  gives  the  same  results  as  the  original. 

Convex  and  concave  templets  or  gauges  are  then  made  of 
the  radii  of  the  various  constituent  parts,  and  pairs  of  tools  are 
next  turned  to  fit  the  curvature  of  the  gauges.  These  tools  are 
divided  into  three  classes — 

Roughers,  True  Tools,  and  Polishers. 

They  are  similar  to  A  and  C  in  diagram  below,  and  are  fixed 
to  a  rotating  spindle  similar  to  that  of  a  lathe,  which,  however, 


Blfc=^  > 


0 


I 


Fig.  27. 

can  be  either  vertical  or  horizontal.  These  tools  must  of  neces- 
sity be  accurately  curved  to  the  spherical  shape  that  the  glass  is 
to  assume  in  lens  form. 

The  Glass. — The  glass  is  received  in  thick  slabs  or  plates,  and 
by  means  of  a  slitting  machine,  consisting  of  a  rapidly  rotating 
iron  plate  charged  with  diamond  dust  and  oil,  thin  plates  are  cut 
off  to  approximately  the  thickness  of  the  lens. 

Peeparation  and  Processes. — These  thin  plates  are  then  cut 
into  small  pieces  by  means  of  a  diamond. 

Hand-shanks  are  next  used  to  take  off  the  square  edges  and 
to  render  the  small  piece  of  glass  nearly  round. 

It  is  then  cemented  to  a  holder  which  rotates  in  the  lathe,  and 
by  means  of  a  sharp  tool  of  steel  and  water  is  edged  to  within  a 
fraction  of  its  ultimate  diameter. 


OPTICAL  CONSTEUCTION  81 

Then,  with  the  same  tool  or  sometimes  with  a  diamond,  the 
face  is  shaped  spherically. 

The  lens  in  its  rough  state  is  now  removed  in  its  holder  from 
the  lathe,  and  the  roughing  tool  takes  its  place.  The  lens  fixed 
to  its  holder  as  in  B  and  D  is  now  held  in  the  hand  ;  the  lathe  is 
rotated,  and  by  means  of  emery,  moistened  with  water,  the  lens 
is  ground  against  its  corresponding  tool  A  or  C  with  a  peculiar 
rotary  movement  of  the  wrist,  and  as  the  shape  of  the  lens 
becomes  more  true,  a  finer  and  yet  finer  emery  is  used  until  the 
'figure' — that  is,  the  ultimate  curve — is  put  upon  it  ready  for 
polishing. 

For  Polishing,  a  tool  lined  with  a  composition  consisting 
largely  of  pitch,  and  moulded  true  to  curve,  is  fixed  and  rotated 
in  the  lathe,  and  the  glass  is  continuously  worked  with  it,  special 
polishing  materials  being  used,  such  as  rouge  or  putty-powder ; 
and  from  time  to  time,  at  the  figuring  stage  and  during  the 
polishing,  the  curve  is  tested  by  means  of  a  '  proof  plate.' 

The  Proof  Plate. — This  proof  plate  consists  of  a  plate  of 
glass  which  has  been  worked  so  as  to  precisely  fit  the  exact 
curve  of  the  lens  which  it  is  intended  to  test  by  its  means.  The 
accuracy  of  the  proof  plates  is  ascertained  by  micrometer  gauge 
and  spherometer. 

If  a  carefully  cleaned  lens  be  brought  into  contact  with  the 
curve  in  the  proof  plate  and  it  is  nearly  correct,  then  the 
phenomena  known  as  '  Newton's  rings '  will  appear.  These 
coloured  rings  are  produced  whenever  two  reflecting  surfaces  are 
brought  very  close  together,  for  they  are  due  to  the  interference 
of  the  light  reflected  from  one  of  the  surfaces  with  that  from  the 
other,  and  the  beautiful  colours  seen  in  soap-bubbles  are  a  well- 
known  example  of  them.  These  rings  form  an  extremely  delicate 
test  for  the  truth  of  lens  surfaces  ;  for,  roughly  speaking,  the 
colours  run  through  the  complete  range  of  the  spectrum  for 
every  increase  of  the  space  between  the  adjoining  surfaces  of 
the  lens  and  proof  plate  by  one  fifty-thousandth  part  of  an  inch  ; 
and,  as  even  moderate  changes  of  tint  in  any  one  colour  are 
easily  perceptible,  it  is  easy  to  see  that  by  this  test  irregularities 
in  the  surfaces  of  so  little  as  one-millionth  part  of  an  inch  can 
be  detected.  When  a  lens  is  absolutely  correct  to  the  proof 
plate,  the  appearance  is  either  that  of  one  uniform  tint  of  colour 


82 


MODEBN  MICROSCOPY 


over  the  entire  surface,  or,  if  the  contact  is  a  little  closer  on  one 
edge  than  on  the  other,  then  straight  bands  of  colour  will  appear. 
Any  difference  of  curvature  between  the  lens  and  proof  plate 
betrays  itself  by  the  appearance  of  rings,  and  if  the  lens  surface 
is  not  truly  spherical  the  rings  become  deformed  from  the  true 
circular  shape  which  they  should  show,  being  elliptical  or  some- 
times even  triangular  in  form. 

Final  Steps.— When  the   surfaces  of   the  lenses  have  thus 
been  figured   true   to  the  proof  plates,  the  lens  is  once  more 


Fig.  28. — Proof  Plate  and  Newton's  Rings. 

mounted  on  the  lathe-spindle  to  be  centred — that  is,  to  have 
a  smooth  edge  turned  and  ground,  perfectly  true  with  the  optical 
axis  of  the  lens. 

The  various  constituents  are  then  cemented  together  and 
baked  for  several  hours,  and  are  subsequently  mounted  in  their 
brass  fittings,  and  the  adjustment  for  axial  truth  and  distance 
completes  the  process. 


EYEPIECES. 

The  eyepiece  commonly  used  with  the  microscope  is  what  is 
termed  the  Huyghenian  form,  which  generally  consists  of  two 
plano-convex  lenses  placed  at  a  distance  apart  about  equal  to 
half  the  sum  of  their  foci,  with  a  stop  in  the  principal  focus  of 
the  eye-lens.  This  will  be  found  to  meet  all  ordinary  require- 
ments of  microscopical  work  with  achromatic  objectives.  Eye- 
pieces vary  in  power,  and  these  powers  are  usually  designated 
by  the  letters  A,  B,  C,  D,  etc.,  A  being  the  weakest  power.  On 
the  Continent  they  are  generally  designated  1,  2,  3,  4,  etc.,  while 
some  firms  express  their  power  in  focal  units ;  for  instance,  an 
eyepiece  having  a  power  of   10  would   be   1  inch.     This  last 


OPTICAL  CONSTRUCTION 


83 


method,  or  that  adopted  by  Zeiss  for  the  compensating  eye- 
pieces, and  several  progressive  English  houses  for  their  ordinary 
eyepieces,  where  the  actual  magnifying  power  is  engraved   on 
the  cap  of  the  eyepiece,  is  the  only  rational  one.     The  letters 
A,  B,  C,  etc.,  or  Nos.  1,  2,  3,  etc.,  convey  no  real  idea  of  the 
magnifying  powers  of  the  eyepieces,  because  each  maker  has  his 
own  formula  for  each  eyepiece,  and  there  is  no  correspondence 
in  the  powers  of  one  eyepiece  marked  '  D '  by  one  maker  and 
that  supplied  by  another.     It  is  often  remarked  that  the  Con- 
tinental objectives  stand  a  stronger  power  of  eyepiece  than  the 
English,  and  on  this  account  a  superiority 
has  been  claimed  for  them ;    but  it  should 
be   borne   in   mind  that  English   manufac- 
turers  give   in   many  instances   as   deep   a 
power  of  eyepiece  as  20  or  25,  whereas  Con- 
tinental  manufacturers  rarely  supply  them 
of  greater  magnifying  power  than  10  or  12 
diameters.     In  the  English  series  the  varia- 
tion in  power  between  two  consecutive  eye- 
pieces is  generally  greater  than  in  the  Con- 
tinental   series.      A   comparison,   therefore, 
between   the   merits   of    an  English  object- 
glass  tested  with,  say,  a  'D'  or  No.  4  English 
eyepiece  and  a  Continental    object-glass  of 
the  same  power  tested  with  a  No.  4  eyepiece 
of  Continental  make  would  not  be  fair,  as  the  former,  having 
a  deeper  power  eyepiece  on  it,  would  be  liable  not  to  give  such 
perfect  results  as  the  latter.     There  is  no  reason  why  a  standard 
series   of    eyepieces   should   not   be   established,    to   which   all 
makers  could  conform ;  it  would  greatly  add  to  convenience  in 
working.     Although  people  very  often  buy  deep-power  eyepieces, 
it  is  advisable,  with  ordinary  achromatic  lenses,  that  no  stronger 
power  should  be  used  than  an  eyepiece  giving  an  initial  power 
of  10  or  12  diameters.     The  best  eyepiece  for  general  purposes 
is  the  '  B.'      This  gives  a  convenient   size  of  field,  and  is  by 
far   the   most   comfortable   to  work  with  of   the  whole  series. 
Next,  and  in  addition  to  this,  we  should  recommend  either  the 
'C  or  the' D.' 
Microscopists  having  abnormal  vision,  and  preferring  to  work 


Fig.  29. — Huyghenian 

Eyepiece. 


84  MODERN  MICROSCOPY 

without  spectacles,  should  have  an  auxiliary  cap  made  to  fit 
over  their  eyepieces,  carrying  a  lens  of  the  power  that  corrects 
the  error  of  vision.  This  is  especially  necessary  where  measuring 
has  to  be  done,  or  where  the  microscope  is  arranged  for  a  second 
person's  inspection. 

At  times  it  is  desired  to  know  whether  an  eyepiece  can  have 
its  diaphragm  enlarged  so  as  to  give  a  larger  field.  An  easy 
method  of  ascertaining  how  much  of  the  field  lens  is  employed 
is  to  make  a  spot  with  ink  near  the  margin  on  the  convex  side 
of  the  field  lens,  and  on  placing  the  eyepiece  in  the  microscope, 
if  the  diaphragm  has  a  sufficiently  large  aperture,  the  ink  will 
be  visible  ;  if  not,  it  may  be  enlarged  until  it  appears.  The 
diaphragm  should  not  be  so  large  as  to  admit  of  more  than  the 
edge  of  the  field  lens  being  visible. 

Compensating  Eyepieces. 

Under  the  description  of  '  Apochromatic  Objectives '  on  p.  55, 
reference  is  made  to  compensating  eyepieces.  These  are  specially 
designed  to  correct  an  outstanding  colour  defect  (of  the  nature 
of  under-correction)  which  is  inherent  in  all  high-power  objectives, 
whether  they  be  apochromatic  or  achromatic,  on  account  of  the 
peculiar  construction  of  the  front  lens.  For  the  sake  of  uni- 
formity of  eyepiece,  Zeiss  imparts  the  same  colour  effect  to  the 
lower-power  lenses  of  the  apochromatic  series.  The  eyepieces, 
then,  have  an  equal  error  of  the  opposite  kind  (over-correction), 
and  when  the  objective  and  eyepiece  are  combined,  a  perfect 
correction  is  obtained. 

The  apochromatic  objectives  are  '  under-corrected,'  while  the 
achromatic  objectives  of  low  power  are  '  over-corrected.'  The 
compensating  eyepieces  for  the  former  are  over-corrected,  and 
the  Huyghenian  eyepieces  for  the  latter  under-corrected.  With 
low  powers  of  the  achromatic  type,  the  compensating  eyepieces 
are  disadvantageous  ;  but  with  high  powers,  where  the  defects 
caused  by  the  hemispherical  front  lens  give  rise  to  error  identical 
with  that  in  the  apochromatic  objectives,  and  which  the  com- 
pensating eyepieces  are  designed  to  overcome,  these  special  eye- 
pieces can  be  employed,  but  the  result  is  not  sufficiently  beneficial 
to  justify  the  purchase  of  them  for  use  with  high-power  achromatic 
objectives  only. 


OPTICAL  CONSTRUCTION 


85 


A  most  advantageous  feature  is  imparted  to  the  Zeiss  com- 
pensating eyepieces.  They  are  all  designed  to  work  in  the  same 
focal  plane,  so  that  when  two  eyepieces  of  this  series  of  different 
powers  are  interchanged  in  the  body  of  the  microscope  no  altera- 
tion in  the  focussing  is  necessary. 

A  number  is  engraved  on  each  eyepiece,  which,  multiplied  by 
the  initial  magnifying  power  of  the  objective,  will,  when  used  at 
the  tube-length  for  which  the  eyepiece  is  designed,  indicate  the 
magnifying  power  that  is  being  employed. 

The  compensating  eyepieces  designed  for  the  6-inch  tube- 
length  can  be  used  on  the  10-inch  tube,  and  those  for  the  10-inch 
at  the  6-inch  tube-length,  without  detrimental  effect ;  but  in  the 
former  case  about  half  must  be  added  to  the  product  of  the 
multiplication  of  the  power  of  the  objective  and  eyepiece,  and  in 
the  latter  case  about  one-third  must  be  deducted,  in  order  to 
arrive  at  the  magnifying  power  (see  ante,  p.  55). 


Holoscopic  and  Universal  Eyepieces. 

These  eyepieces  are  made  by  J.  Swift  and  Son  and  Watson 
and  Sons,  and  are  intended  to  be  used  with  objectives  of  both 
the  apochromatic  and  achromatic  types. 
The  lenses  used  are  made  of  a  selected 
optical  glass,  which  produces  a  degree  of 
over  -  correction  similar  to  that  associated 
with  compensating  eyepieces  when  the 
separation  between  the  eye  and  field  lenses 
is  increased.  In  order  that  this  may  be 
conveniently  effected  the  eye-lens  is  attached 
to  an  inner,  or  draw-tube,  sliding  inside  the 
outer  tube  which  fits  the  microscope  body. 
When  the  eyepiece  is  closed  together  it 
becomes  of  the  Huyghenian  type ;  when 
the  eye-lens  tube  is  pulled  out,  it  gives  the 
effect  of  a  compensating  eyepiece.  The 
amount  of  over -correction  can  be  exactly 
obtained  by  the  greater  or  lesser  extension  of  its  draw-tube, 
a  scale  being  provided  to  record  results.  This  eyepiece  is, 
therefore,  applicable  to  all  classes  of  objectives,  and,  being  made 


Fig. 


30. — Holoscopic 
Eyepiece. 


86  MODERN  MICROSCOPY 

in  a  useful  range  of  magnifications,  will  be  found  a  desirable 
pattern  to  start  with,  and  acquaintance  with  its  working  will 
lead  to  greater  appreciation  of  it.  It  yields  excellent  effects 
photographically.  Even  if  it  be  intended  to  limit  the  equipment 
to  achromatic  objectives,  this  type  of  eyepiece  will  generally  be 
found  to  present  points  of  superiority  over  the  ordinary  Huy- 
ghenian  pattern. 

Kellner  Eyepieces. 

This  is  an  achromatic  form  of  eyepiece,  giving  an  exceedingly 
large  field,  which  is  considerably  used  for  the  examination  of 
anirualculae,  pond  life,  etc.  A  certain  amount  of  definition  is, 
however,  sacrificed  in  working  with  it ;  and  although  occasion- 
ally of  use,  we  should  recommend  the  microscopist,  before 
purchasing,  to  judge  for  himself  as  to  the  desirability  or  other- 
wise of  his  having  them.  They  are  not  by  any  means  necessary 
adjuncts. 

Projection  Eyepieces. 

These  were  designed  specially  for  projecting  objects  on  a  screen 
and  for  photographic  purposes.     They  give  an  exceedingly  small 

field,  but  an  exquisitely  sharp  one.     In  order 

fto  obtain  good  results  with  them,  it  is  neces- 
sary to  alter  the  position  of  the  eye-lens  until 
the  image  of  the  diaphragm  appears  sharply 
projected  upon  the  screen.     For  this  purpose 
the  eye-lens  is  mounted  in  a  tube  having  in 
it   a    spiral    slot,  permitting  of   the   eye-lens 
being  moved  to  and  fro  with  great  precision. 
They  are  usually  made  in  four  powers,  mag- 
nifying   2    and   4   diameters   respectively  for 
the  short  tube-length,  and  3  and  6  diameters 
for  the  English  tube-length.    The  most  service- 
able are  the  4  for  the  short  tube  and  the  3  for 
EyepieckCTI0N  ^ne  l°no    tube.      All   photo-micrographers  of 
note  use  these  eyepieces,  and  they  can  usually 
be   employed   to   advantage,   even    with   low-power   achromatic 
objectives  of  good  quality.     For  photographing  with  ordinary 
objectives  of  low  power,  the  '  A '  eyepiece  gives  good  results. 


OPTICAL  CONSTRUCTION  87 

Binocular  Eyepieces. 

As  mentioned  under  '  Binocular  Microscopes,'  neither  of  the 
leading  Continental  firms  make  the  Wenham  binocular  micro- 
scope. For  those,  therefore,  who  desire  to  be  able  to  employ 
both  eyes,  they  make  a  binocular  eyepiece,  suitable  for  both 
high  and  low  powers,  but  only  for  the  Continental  length  of 
tube,  and  this  should  be  particularly  understood.  The  one  with 
which  we  are  acquainted  is  that  designed  by  Abbe  and  manu- 
factured by  Zeiss,  as  shown  in  Fig.  32.  In  some  hands  it  gives 
very  beautiful  results,  while  other  workers  have  failed  to  derive 
advantage  from  it.  It  is  designed  to  give  stereoscopic  effects 
and  to  work  with  both  high  and  low  powers.  If  it  were  mounted 
in  some  lighter  manner  it  would  perhaps  become  more  generally 
used  ;  its  weight  is  very  much  against  it  when  working  with 
high  powers  ;  still,  for  the  advantages  it  affords  it  is  an  adjunct 
which  is  by  no  means  to  be  despised. 

Blank  Eyepiece. — It  will  be  found  convenient,  especially  in 
examining  the  back  of  the  objective  to  observe  diffraction  phe- 
nomena, cones  of  illumination,  etc,  that  a  blank  or  'dummy' 
eyepiece  be  employed ;  that  is,  an  ordinary  eyepiece  mount 
having  no  lenses  in  it.  The  aperture  in  the  cap  must,  however, 
be  a  very  small  one. 

STANDARD  GAUGES  FOR  EYEPIECES. 

The  following  sizes  were  adopted  by  the  Council  of  the  Royal 
Microscopical  Society  on  December  20, 1899,  as  the  standard  inside 
diameters  of  draw-tubes  for  microscopes,  the  tightness  of  the  fit  of 
the  eyepiece  being  left  to  the  discretion  of  the  manufacturers  : 

No.  1,  0*9173  inch  -  23*300  millimetres. 

No.  2,  1-04  inches  =  26*416 

No.  3,  1*27  inches  =  32*258 

No.  4,  1-41  inches  =  35*814 

No.  1  is  what  is  known  as  the  Continental  size,  which  is  made 
almost  universally  by  Continental  manufacturers,  and  has  been 
supplied  for  many  years.  It  has  also  been  largely  used  by 
English  manufacturers. 

No.  2  is  the  mean  of  the  sizes  used  by  the  English  trade  for 
students'  and  small  microscopes. 


88 


MODERN  MICROSCOPY 


No.  3  is  the  mean  of  the  sizes  used  for  medium- sized  bin- 
oculars and  for  microscopes  of  a  similar  class. 

No.  4.  is  the  maximum  size  for  long  tube  binoculars. 

Two  sizes  only  of  these  are  in  general  use — viz.,  Nos.  1  and  3, 
and  these  appear  to  meet  all  requirements.  Notwithstanding 
the  choice  given  by  the  society,  certain  houses  for  reasons  of 


Fig.  32. — Zeiss's  Binocular  Eyepiece. 


their  own  make  draw-tube  fittings  of  diameters  which  do  not 
conform  to  either  of  the  above-mentioned  standards,  and  those 
who  may  be  contemplating  the  purchase  of  a  microscope  will.be 
conferring  a  benefit  on  microscopy  generally  and  directly  on 
themselves  if  they  insist  that  the  microscope  they  select  shall 
have  one  or  other  of  the  standard  sizes  of  fittings  for  eyepieces, 
preferably  No.  1  or  No.  3,  and  refuse  to  accept  any  other 
diameter. 


CHAPTER  III 
ILLUMINATION  AND  ILLUMINATING  APPARATUS 

Monochromatic  Light  and  Light  Filters. 

Absolutely  monochromatic  light  is  a  light  of  one  refrangibility 
— that  is,  a  colour  of  one  uniform  wave-length.  As  used  in 
microscopy,  monochromatic  light  means  light  with  a  small  range 
of  refrangibility,  and  it  is  important  that  its  function  should  be 
clearly  understood. 

If  white  light  is  divided  into  its  component  parts  by  means  of 
a  prism  or  a  spectroscope,  a  regular  band  of  colours  is  produced, 
termed  the  spectrum,  commencing  with  red  at  one  end,  followed 
by  orange,  yellow,  green,  blue,  indigo,  and  finishing  with  violet. 

In  physical  optics  light  is  regarded  as  travelling  in  waves,  the 
amplitude  of  each  of  which  is  very  small,  compared  with  the 
wave-length — not  more  than  about  1  :  10,000.  Now,  the  length 
of  a  light  wave  varies  according  to  the  portion  of  the  spectrum 
that  is  used.  At  the  extreme  red  end  of  the  spectrum  it  measures 
0*76/v:and  the  wave-length  decreases  through  the  range  of  colours 
until  at  the  extreme  violet  end  it  measures  0*39  /x.  From  this  it 
will  be  seen  that  nearly  double  the  number  of  waves  of  light 
would  be  oscillating  per  millimetre  with  a  violet  light  than  with 
a  red. 

The  numerical  aperture  of  an  objective  is  increased  by  the  use 
of  a  dense  medium  enveloping  the  object  and  the  front  lens  of 
the  objective,  as  we  have  seen  by  the  description  of  immersion 
objectives.  The  oil  or  other  medium  employed  shortens  the 
wave-length  of  the  light  used,  whatever  its  colour,  and  when  we 

*  /j.=T-J-JJ  of  a  millimetre,  and  is  called  a  micron.     There  are  7^  to  8 
microns  in  the  diameter  of  a  human  blood-disc. 

89 


90  MODERN  MICROSCOPY 

use  a  light  of  shorter  wave-length  than  it  would  have  when  passing 
through  air,  we  increase  the  effective  aperture  of  the  objective. 
Accordingly  the  resolving  power  of  a  lens  is  increased  by  shorten- 
ing the  wave-length  of  the  light  admitted  to  it,  and  this  is 
accomplished  in  either  of  two  ways — (1)  By  employing  blue 
instead  of  white  light,  or  (2)  by  converting  the  lens  into  an 
immersion  lens,  and  interposing  a  layer  of  oil  instead  of  air 
between  it  and  the  object.  For  instance,  if  a  microscopic  objec- 
tive were  used  with  white  light,  and  its  limit  of  power  to  resolve 
fine  structure  were  50,133  lines  per  inch  with  such  illumination, 
its  limit  would  be  54,342  lines  per  inch  with  monochromatic  blue 
light  (line  F). 

A  natural  conclusion  from  these  statements  would  be  that  the 
farther  towards  the  violet  the  monochromatic  light  were  used, 
the  more  marked  would  be  the  results  obtained  ;  but  although 
this  is  correct  theoretically,  it  is  not  true  practically.  Microscope 
objectives  are  corrected  for  visual  purposes  for  use  with  the 
brightest  rays  of  white  light,  and  if  the  extreme  ends  of  the 
spectrum  were  employed — the  objective  not  being  calculated  for 
these — rise  would  be  given  to  spherical  aberration,  even  in  the 
best  objectives,  preventing  the  accomplishment  of  good  work. 
If  a  lens  were  corrected  for  spherical  aberration  when  used 
with  light  from  the  extreme  blue  end  of  the  spectrum,  under 
existing  conditions  of  manufacture  it  would  work  at  its  best  with 
the  light  for  which  it  was  designed,  and  if  light  lower  down  in  the 
spectrum  were  employed,  spherical  aberration  would  be  apparent. 
It  must  be  borne  in  mind  that  light  of  extremely  short  wave- 
length is  sensibly  absorbed  by  glass,  also  the  eyesight  is  not  keen 
in  extreme  blue  and  violet  lights,  consequently  the  range  of  light 
that  is  practically  available  for  monochromatic  illumination  is 
restricted. 

Another  advantage  gained  by  the  use  of  monochromatic  light  is, 
that  as  there  is  but  one  colour  of  the  spectrum  used,  objectives 
of  high-class  make  of  the  achromatic  form  are  rendered  practically 
equal  to  apochromatics  by  the  removal  of  the  secondary  spectrum 
— that  is,  any  chromatic  aberration  that  may  be  present  in  the 
objective  is  annulled  by  the  monochromatic  light  with  which  the 
illumination  is  effected.  The  more  nearly  the  monochromatic 
light  which  is  used  approaches  to  that  ray  for  which  the  spherical 


ILLUMINATION  AND  ILLUMINATING  APPARATUS     91 

aberration  in  the  objective  is  best  corrected,  the  better  will  be 
the  resulting  definition.  In  some  cases,  so  advantageous  has 
this  means  of  illumination  proved,  that,  using  two  objectives  of 
the  same  power  alternately,  one  an  expensive  apochromatic,  and 
the  other  an  achromatic,  it  has  been  difficult  to  tell  which  was 
being  employed. 

True  monochromatic  light  can  be  at  the  present  time  obtained 
by  means  of  prisms  only,  and  the  most  practical  apparatus  that 
is  known  to  us  is  that  designed  by  Dr.  Spitta,  and  manufactured 
by  Mr.  C.  Baker,  of  244,  High  Holborn,  London.  It  consists  of  a 
prism  with  a  diffraction  grating,  slits,  and  condensing-lenses  so 
arranged  as  to  afford  every  facility  for  obtaining  accurate  effects. 

Beautiful  results  with  monochromatic  light  are  to  be  secured 
by  the  use  of  a  heliostat  to  reflect  sunlight  upon  a  slit  and 
prisms. 

Dr.  G.  Johnstone  Stoney  devised  a  most  excellent  heliostat 
actuated  by  means  of  a  lever  clock  movement,  and  with  fine 
adjustments  by  means  of  Hook's  handles  for  setting,  and  a  plane- 
parallel  worked  mirror.  The  brilliance  of  illumination  by  such 
means  is  very  intense,  and  a  spectrum  2  feet  in  length  can 
be  easily  secured  if  the  microscope  be  placed  at  some  distance 
from  the  prisms.  If  the  spectrum  be  allowed  to  fall  on  a  sheet 
of  white  paper  or  cardboard  at  the  position  in  which  the  micro- 
scope is  placed,  the  exact  tint  of  illumination  that  is  required 
can  be  selected,  and  that  alone  utilized  in  the  microscope.  The 
use  of  a  heliostat  is  necessarily  limited  in  Great  Britain,  and 
especially  so  in  London,  on  account  of  the  few  hours  of  bright 
sunlight  that  are  available ;  but  in  countries  where  this  restriction 
does  not  apply  the  heliostat  is  particularly  to  be  recommended, 
not  only  for  this  especial  purpose,  but  also  for  general  microscop- 
ical work  and  in  photography. 

Many  experiments  have  been  made  with  a  view  to  producing 
monochromatic  light  screens  by  means  of  pigments  and  the 
combination  of  coloured  glasses,  and  thereby  obviating  the 
necessity  for  prisms.  So  far  these  attempts  have  been  only 
partially  successful,  all  that  have  been  made  passing  light  of 
more  than  one  colour.  The  best  results  have  been  achieved 
by  Messrs.  Wratten  and  Wainwright,  who  supply  for  photo- 
micrography especially  a  series  of   excellently  corrected  colour 


92  MODERN  MICROSCOPY 

screens,  or  light  filters,  giving  almost  every  desired  colour  effect. 
Very  material  assistance  is  afforded  by  these  screens,  not  alone 
for  actual  monochromatic  illumination,  but  also  for  general  work, 
for  they  allow  a  large  cone  of  illumination  to  be  utilized  without 
discomfort  to  the  eyes.  They  are  usually  placed  beneath  the 
sub-stage  condenser,  and  are  employed  in  photo-micrography 
very  considerably  for  neutralizing  non-photographic  colours  in 
objects,  and  rendering  the  actinic  and  visual  rays  in  an  objective 
more  nearly  coincident ;  also  in  visual  work  for  minimizing  light 
glare  with  large  cones  of  illumination.  This  latter  applies  equally 
to  apochromatic  and  achromatic  objectives,  the  screen  often  pro- 
ducing stronger  contrast  and  a  crisper  image  than  could  be 
obtained  without  one. 

The  most  advantageous  colour  for  all-round  visual  purposes  is 
the  green-blue,  and  most  effective  light  screens  are  made  on  a 
plan,  originally  suggested  by  Mr.  J.  W.  Gifford,  consisting  of 
a  film  of  gelatine  stained  with  malachite  green  deposited  on  a 
circular  disc  of  signal  green  glass,  and  having  a  protecting 
cover-glass  for  the  gelatine  fixed  by  means  of  a  ring  of  cement. 
Fluid  screens  can  also  be  made  with  great  accuracy,  and  with 
care  will  last  a  considerable  period,  but  it  is  particularly  essen- 
tial that  they  be  protected  from  light,  or  they  are  liable  to  fade. 
The  Gifford's  screen,  referred  to  above,  in  a  fluid  form  is  made 
by  mixing  a  small  proportion  of  malachite  green  in  glycerine  in 
a  trough.  The  light  from  the  illuminant  that  is  to  be  used  is 
examined  spectroscopically  through  the  medium  in  the  trough, 
and  the  coloured  fluid  is  added  until  the  red  end  of  the  spectrum 
is  absorbed ;  if  this  be  done  exactly  a  minimum  of  loss  of  light 
occurs. 

Another  excellent  screen  is  produced  by  making  a  saturated 
solution  of  acetate  of  copper,  but  a  trough  with  an  opening  of 
at  least  J  inch  back  to  front  is  necessary  to  obtain  an  effective 
colour  with  this  fluid. 

For  photography,  discs  of  light  and  dark  yellow  and  various 
shades  of  blue  glass  are  constantly  employed,  and  particularly 
Dr.  Spitta's  '  pot'  green  glass  disc,  which  is  useful  for  all  purposes, 
is  an  inexpensive  and  effective  light  filter.  The  great  desideratum 
in  a  light  screen  is  that  it  shall  pass  a  large  quantity  of  light. 
Mixtures  could,  probably,   soon   be  made   that  would  produce 


ILLUMINATION  AND  ILLUMINATING  APPARATUS     93 

monochromatism,  if  only  great  transparency  were  not  of 
importance. 

Mr.  Nelson  states*  that  monochromatic  blue  light  'makes  a 
difference  of  about  14  per  cent,  in  the  case  of  low  apertures,  but 
beyond  those  of  0*9  N.A.  its  influence  in  increasing  resolution 
is  so  small  as  to  be  hardly  worth  taking  into  account.  What  it 
does  effect  is  the  sharpening  and  clearing  of  detail  already 
resolved.' 

This  is  no  doubt  because  only  a  small  part  of  the  first  dif- 
fraction image,  seen  when  looking  at  the  back  of  the  objective 
with  the  eyepiece  removed,  is  utilized  by  the  lens,  and  that  this 
part  consists  of  blue  light  whether  it  be  blue  or  white  light  that 
is  employed  for  illuminating,  so  that  in  both  cases  there  are  the 
same  materials  for  resolving  the  detail  of  the  object,  and  the  only 
difference  is  that  there  is  a  haze  of  light  of  lower  refrangibility 
(or  greater  wave-length)  also  present,  which  forms  a  luminous 
veil  over  the  whole  field.  It  is  this  veil  which  is  removed  by 
using  monochromatic  light,  and  therefore  the  effect  is  to  sharpen 
and  clear  the  detail  that  is  already  resolved. 

Sub-Stage  Condensers. 

A  few  years  since  all  that  was  considered  necessary  in  order 
to  illuminate  in  a  proper  manner  an  object  under  examination 
was  the  mirror,  perhaps  in  conjunction  with  the  bull's-eye  stand 
condenser ;  and  in  many  cases  the  mirror  was  hung  on  a  tail- 
piece which  could  be  moved  in  an  arc  round  the  centre  of  the 
stage,  and  by  this  means  light  at  any  angle  could  be  reflected  on 
the  object.  The  day  for  this,  however,  has  gone  by,  and  anyone 
who  requires  to  get  even  fair  results  must  use  a  sub-stage  con- 
denser in  some  form  or  other.  Especially  does  this  apply  to 
high-power  objectives.  Plenty  of  illumination  can  be  obtained 
with  the  mirror  only  for  low-power  objectives,  but  beyond  these 
the  object  becomes  ill-defined  and  the  field  dark.  More  especially 
since  the  study  of  bacteriology  has  taken  so  prominent  a  position 
has  the  condenser  come  to  the  front.  Without  its  aid  it  would 
be  almost  impossible  to  distinguish  between  different  species  of 
these  minute  organisms.     To  the  leading  members  of  the  Royal 

*  Journal  of  the  Royal  Microscopical  Society,  1893,  p.  15. 


94  MODERN  MICROSCOPY 

Microscopical  Society,  and  especially  to  the  late  Dr.  Dallinger 
and  Mr.  E.  M.  Nelson,  is  due  the  steady  improvement  that  has 
taken  place  in  the  optical  qualities  of  the  sub-stage  condenser. 
The  two  gentlemen  named  were  indefatigable  in  their  appeals 
and  demonstrations  to  microscopists,  urging  the  pre-eminent 
position  that  it  should  occupy  in  manipulation,  and  the  proper 
methods  of  using  it. 

One  of  the  most  largely  used  of  condensers  is  a  chromatic  one, 
named  the  Abbe  illuminator,  originated  by  the  firm  of  Carl  Zeiss, 
and  now  supplied  by  nearly  all  opticians.  It  is  made  in  two 
forms,  one  having  a  numerical  aperture  of  1*20,  and  the  other 
of  1*40.  The  former  is  the  more  commonly  employed,  and  is 
principally  adopted  by  students.  The  optical  portion  is  shown 
in  Fig.  33.     It  gives  a  brilliant  illumination,  with  the  highest 


Fig.  33.— Abbe  Illuminator.     (l-20  N.A.,  Optical  Paet.) 

power  objective,  while  with  the  lower  powers,  by  removing  the 
top  lens,  good  results  can  be  obtained.  Its  price  is  low,  but  it 
has  the  great  disadvantage  of  not  being  achromatic,  and  having 
so  large  an  amount  of  spherical  aberration  as  to  be  almost 
useless  for  critical  work,  for  although  its  total  aperture  is  large, 
its  aplanatic  aperture  is  less  than  0*50.  Its  popularity  is  to  a 
great  extent  due  to  the  facility  with  which  it  can  be  used,  in 
consequence,  probably,  of  its  large  field  lens  and  its  want  of 
definite  focus.  Nevertheless,  it  fills  a  distinct  position  in 
microscopical  work. 

Modern  critical  research  work  necessitates  the  use  of  the  best 
sub-stage  condensers  procurable  ;  consequently,  the  day  for  the 
chromatic  condenser  is  passing,  excepting  for  work  of  an  unim- 
portant nature,  and  English  manufacturers  have  provided  for 
every  requirement  in  an  ample  manner.  The  importance  of  a 
well-corrected  condenser  can  best  be  understood  by  the  effect 
produced  on  the  working  aperture  of  the  objective.     To  ascertain 


ILLUMINATION  AND  ILLUMINATING  APPAEATUS    95 

this  the  aplanatic  aperture  of  the  condenser  and  the  numerical 
aperture  of  the  objective  should  be  added  together,  and  divided 
by  two,  thus  :  If  an  objective  of  1*30  N.A.  is  used  with  an  Abbe 
illuminator  having  an  aplanatic  cone  of  0*50,  an  effective  working 
aperture  is  obtained  of  0*90  (l,30  +  0,50-f-2).  If  the  same  ob- 
jective is  used  with  a  well-corrected  condenser  having  an  aplanatic 
aperture  0*90,  an  effective  working  aperture  of  1*10  is  obtained 
(1-30  +  0-90^2). 

Swift  and  Son  make  a  range  of  condensers  suitable  for  all 
powers,  and  the  excellent  Holoscopic  series  of  condensers,  com- 
puted by  Mr.  Conrady  and  manufactured  by  Watson  and  Sons, 
are  well  known.     Special  mention  should  be  made  of  the  Uni- 


Fig.  34. — C.  Baker's  Achromatic  Condenser. 


versal  and  the  oil  immersion  of  the  latter  series;  these  are 
constructed  on  the  principle  of  the  Holoscopic  objectives,  and 
have  aplanatic  apertures  closely  approaching  their  total  aper- 
tures. The  Universal  has  a  large  back  lens  0*92  inch  diameter, 
an  aplanatic  aperture  extending  0*90,  and  the  equivalent  focus 
of  TV  inch.  This  power  and  aperture  are  the  most  generally 
useful  that  can  be  desired,  and  if  the  upper  lens  is  removed  by 
unscrewing,  a  condenser  of  longer  focus  is  available  for  use  with 
low-power  objectives.  It  is  the  best  possible  for  medium  powers 
(J  inch),  and  with  oil  immersion  objectives  is  generally  sufficient. 
It  is  only  when  oil  immersion  objectives  of  large  numerical  aper- 
ture are  to  yield  their  maximum  effect  that  the  oil  immersion 
condensers  are  essential. 

The  achromatic  condenser  by  Pi.  and  J.  Beck,  with  the  total 
aperture  of  1*0,  and  an  aplanatic  aperture  of  0*90,  is  an  excellent 
one,  as  is  also  their  immersion  condenser. 


96 


MODEEN  MICROSCOPY 


Messrs.  Powell  and  Lealand  are  entitled  to  special  commenda- 
tion for  their  early  appreciation  of  the  value  of  condensers  having 
large  aplanatic  apertures,  and  they  were  many  years  in  advance 
of  other  makers  in  the  production  of  such  condensers.  They 
designed  two  of  apochromatic  form,  one  having  a  N.A.  of  1*0  for 
use  dry  and  the  other  an  oil  immersion  having  a  N.A.  of  1'40. 
Some  of  the  condensers  before  referred  to  possess  aplanatic 
apertures  slightly  in  excess  of  those  by  Powell  and  Lealand,  but 
they  are  beautifully  corrected,  and  those  who  may  be  wishing  to 
have  condensers  in  keeping  with  their  apochromatic  objectives 
would  find  these  admirable.     Still,  the  value  of  a  sub-stage  con- 


Fig.  35. — Iris  Diaphragm  as  fitted  to  the  Coxdexser  Carrier. 


denser  is  not  to  be  reckoned  by  its  total  numerical  aperture,  but 
by  the  solid  cone  that  it  will  transmit,  or,  in  other  words,  by 
its  perfection  of  correction  ;  for  its  aplanatic  cone  alone  can  be 
employed  for  critical  illumination. 

If,  therefore,  an  achromatic  condenser  gives  a  superior  aplanatic 
cone  to  an  apochromatic  for  practical  work,  it  may  generally  be 
considered  preferable,  and,  seeing  that  this  is  done,  and  that  the 
achromatic  form  is  very  much  less  costly  than  the  apochromatic, 
the  microscopist  may  with  assurance  take  the  former,  and  be 
satisfied  that  he  can  perform  with  it  all  that  possibly  can  be 
done. 

In  Fig.  34  the  achromatic  condenser  that  is  shown  is  mounted 
on  a  carrier  for  the  sub- stage.  It  is  provided  with  an  iris 
diaphragm  similar  to  that  illustrated  in  Fig.  35,  by  means  of 


ILLUMINATION  AND  ILLUMINATING  APPARATUS     97 

which  any  desired  aperture  may  be  quickly  and  exactly  obtained. 
It  will  be  found  of  utility  to  have  the  arc,  through  which  the 
lever  controlling  the  iris  diaphragm  travels,  provided  with  a  scale 
of  divisions,  so  that  results  may  be  quickly  reproduced  or  any 
special  aperture  may  be  obtained  ;  but  for  this  purpose  it  is 
necessary  that  the  diaphragm  shall  respond  immediately  on  the 
pressure  of  the  lever  handle  ;  there  must  be  no  loss  of  time  in 
the  movement.  The  carrier  is  further  provided  with  an  arm, 
having  a  rotating  cell,  in  which  may  be  placed  stops  for  pro- 
ducing dark-ground  illumination  in  the  same  manner  as  with  the 
spot  lens,  described  on  p.  107  ;  also  stops  for  obtaining  oblique 
illumination  for  the  resolution  of  the  markings  on  diatomaceae, 
and  for  holding  tinted  glasses  or  light  screens.  This  form  of 
carrier  is  applicable  also  to  the  Abbe  illuminator. 

A  very  efficient  condenser  can  be  oftentimes  formed  by  fitting 
a  low-power  objective  into  a  suitable  carrier  in  the  sub-stage. 
A  small  iris  diaphragm,  called  the  Davis's  Shutter,  having  the 
universal  male  Society's  screw  at  one  end  and  a  female  screw  at 
the  other,  together  with  a  tube,  fitting  into  the  sub-stage  and 
provided  with  the  universal  thread,  into  which  the  iris  diaphragm 
may  be  fitted,  is  a  very  suitable  carrier  for  the  objective. 

The  Aplanatic  Aperture  of  a  sub-stage  condenser  is  ascer- 
tained in  the  following  manner  : 

The  condenser  is  accurately  centred,  and  both  it  and  an 
objective  are  focussed  in  the  usual  way  on  an  object  mounted 
in  Canada  balsam,  the  edge  of  the  lamp-flame  being  employed. 
We  will  presume  that  the  objective  has  a  numerical  aperture 
of  0*5.  The  full  aperture  of  the  condenser  is  then  used,  and 
the  object  so  placed  that  the  balsam  portion  is  still  between 
the  condenser  and  objective,  but  the  object  itself  is  not  in  the 
field.  It  would  be  well  that  a  balsam-mounted  object  were 
always  used  for  this  experiment,  as  the  result  is  slightly  affected 
by  different  media.  The  eyepiece  is  now  removed,  and  the  back 
lens  of  the  objective  is  examined.  It  may  be  found  that  it  is 
completely  filled  with  light,  as  in  Fig.  36,  under  which  circum- 
stance the  condenser  has  an  aplanatic  cone  exceeding  the  N.A. 
of  the  objective.  The  aperture  of  the  condenser  can  now  be 
limited  by  means  of  a  diaphragm,  and  an  approximate  value 
obtained  for  the  size  of  diaphragm  that  is  used.     The  edge  of 

7 


98 


MODERN  MICROSCOPY 


this  diaphragm  should  be  so  set  that  its  edge  is  just  seen  appear- 
ing at  the  margin  of  the  objective,  as  in  Fig.  37.  The  aperture 
of  the  condenser  when  used  with  this  size  of  diaphragm  is  there- 
fore a  shade  less  than  N.A.  0'5 — say  0'45.  An  objective  having  a 
larger  N.A.,  say  0*95,  is  now  employed,  and  it  will  be  found  that 
the  back  lens  of  the  objective  is  no  longer  filled  with  light. 
Theoretically,  this  is  the  condition  under  which  the  aplanatic 
aperture  should  be  estimated,  but  when  a  flat  flame  of  a  lamp  is 
presented  edgewise,  its  image  has  corresponding  depth,  and  when 
one  part  is  focussed  on  the  object,  other  parts  of  the  image  of  the 
flame  will  necessarily  be  out  of  focus.  There  is  therefore  a  certain 
range  of  adjustment  of  the  condenser  within  which  the  effect 
(so  far  as  it  depends  on  focussing  the  light  on  the  object)  will 


Fig.  36. 


Fig.  37, 


Fig.  38. 


Fig.  39. 


be  pretty  much  the  same.  But  these  different  positions  give 
different  apertures  to  the  condenser  as  judged  by  the  light 
reaching  the  back  lens  of  the  objective.  The  condenser  should 
then  be  gently  racked  upward  until  the  disc  of  light  is  at  its 
largest  (Fig.  38)  ;  until  on  a  further  movement  of  the  condenser 
two  black  spots  appear,  one  on  either  side  of  the  middle  of  the 
disc  (Fig.  39),  which  increase  as  the  condenser  is  further  racked  up. 
The  last  point  before  the  appearance  of  the  black  spots  furnishes  the 
position  in  which  the  condenser  has  the  largest  aperture  consistent 
with  its  outstanding  spherical  aberration  not  too  much  interfering 
with  the  highest  results,  and  is  the  limit  of  the  condenser  for  critical 
work.  Any  further  advance  of  the  condenser  gives  merely  annular 
illumination,  which,  of  course,  is  to  be  avoided,  excepting  when 
stops  are  used.* 


*  E.  M.  Nelson,  English  Mechanic,  November  16,  1888  (vol.  xlviii.,  No. 
1234),  and  '  The  Microscope  and  its  Revelations,'  by  Dr.  Dallinger. 


ILLUMINATION  AND  ILLUMINATING  APPARATUS     99 


How  to  Use  the  Condenser. 

The  condenser  requires  almost  as  much  care  and  skill  in 
adjusting  as  the  objective,  for  if  it  be  improperly  set  up  it  will 
give  rise  to  '  false  images.'  For  an  objective  to  work  at  its  best 
the  rule  generally  followed  is  to  focus  the  image  of  the  edge  of 
the  lamp-flame  sharply  upon  the  object  on  the  stage,  and  this, 
with  the  modern  condensers  of  large  aperture,  requires  to  be 
as  accurately  performed  as  the  focussing  of  the  objective  upon  the 
object;  hence  the  value  of  the  fine  adjustment  to  the  sub-stage 
(see  p.  18).     The  following  will  be  the  procedure  : 

A  proper  microscope  lamp,  as  described  on  p.  116,  should 
be  set  in  front  of  the  instrument  with  the  edge  of  the  wick  to- 
wards the  microscope,  and  the  light  from  the  lamp  may  be 
allowed  to  fall  directly  upon  the  condenser,  or  a  plane  mirror 
may  be  used.  The  sub-stage  condenser  should  now  be  centred, 
first  having  been  placed  in  about  the  position  that  it  will  occupy 
when  focussed.  The  centring  cannot,  of  course,  be  properly 
effected  without  a  centring  sub- stage ;  but  where  there  is  only 
a  fixed  under-fitting,  it  is  well  to  set  the  condenser  at  the 
position  where  it  is  most  central.  It  is  understood  that  the 
under-fitting  is  centred  with  the  optical  axis  of  the  microscope 
when  sent  out  by  the  makers  ;  but  owing  to  the  fitting-tubes 
being  more  or  less  elliptical,  it  often  happens,  if  the  condenser 
is  rotated  in  the  under-fitting,  that  it  will  be  central  in  one 
position  only,  and  at  this  position  it  should  be  placed  for 
working  when  there  are  no  centring  screws.  Some  condensers 
have  fitted  on  top  of  them  a  removable  cap  with  a  very  small 
pin-hole.  This  pin-hole  should  be  focussed  with,  say,  an  inch 
or  a  J-inch  objective,  and  the  condenser  centred  by  it ;  the 
cap  should  then  be  removed  from  the  condenser  for  working. 
Condensers  not  provided  with  this  cap  can,  as  a  rule,  be  centred 
by  using  a  diaphragm  having  a  very  small  aperture  at  the  back 
of  the  lenses,  and  focussing  the  aerial  image  of  it  with  a  ^-inch 
objective  ;  for  this  reason  the  iris  diaphragm  of  the  condenser 
carrier  should  be  exactly  axial  with  the  condenser  itself.  This 
is  necessary,  too,  for  accurate  working ;  but  the  easiest  way  of 
centring  is  to  make  a  very  small  spot  in  the  middle  of  the  top  of 


100  MODERN  MICROSCOPY 

the  lens  with  a  pen  and  ink  ;  centre  by  this  spot,  and  wipe  it  off. 
It  will  not  make  any  difference  to  the  performance  of  the 
condenser,  and  will  insure  accuracy  and  save  time.  Having 
centred  the  condenser,  it  should  be  racked  up  until  it  touches 
the  under  side  of  the  slide,  the  objective  being  made  to  touch 
the  cover-glass  of  the  object  on  the  upper  side  ;  see  that  the 
diaphragm  of  the  condenser  is  open,  reflect  the  light  with  the 
mirror,  and  thus  illuminate  the  field  ;  then  rack  the  microscope 
body  upwards  until  the  object  comes  into  view.  If  it  is  found 
that  there  is  too  bright  a  flood  of  light,  the  aperture  of  the 
condenser  must  be  decreased  a  little  by  using  a  smaller  diaphragm. 
Having  focussed  the  object  on  the  upper  side  with  the  objective, 

it  will  be  necessary  to  focus  the  condenser. 
Rack  this  downwards  from  the  object  very 
slightly  until  the  image  of  the  lamp-flame 
is  seen  in  the  centre  of  the  field,  the  re- 
mainder being  comparatively  dark,  as  in 
Fig.  40.  If  now  it  be  desired  to  have  the 
whole  field  equally  brilliant,  a  bull's-eye 
stand  condenser  may  be  interposed  be- 
tween the  lamp  and  the  mirror,  the  plane 

Fig.  40. — Image  of  «j        *    j.t_     i_    m  i     •  j      xi 

Lamp-Flame.  sic*e  °*    ^ne   Du^  s_eye   being  towards   the 

lamp  ;  or  the  burner  of  the  lamp  may 
be  turned  round  till  the  flat  of  the  wick  is  towards  the  mirror. 
Where  high  powers  are  to  be  used,  the  object  to  be  examined 
may  with  advantage  be  set  upon  the  stage  and  focussed  with  the 
^-inch  or  other  low-power  objective,  and  the  sub-stage  condenser 
focussed  upon  the  object.  The  high  power  may  then  replace  the 
low  power,  and  the  condenser  will  be  in  adjustment.  If  it  be 
found  that  the  image  of  the  lamp-flame  is  not  in  the  middle 
portion  of  the  field  on  exchanging  the  objectives,  it  will  show 
that  the  objectives  have  not  exactly  the  same  centres,  and  the 
image  must  be  set  central  with  the  high  power  by  altering  the 
position  of  the  condenser  by  means  of  the  centring  screws  of  the 
sub-stage. 

Later  experience  has  shown  that  the  method  recommended 
by  Mr.  Conrady,  and  so  ably  demonstrated  and  advocated  by 
Dr.  Spitta,  has  distinct  advantages.  It  does  not  regard  so  much 
the  sharpness  of  the  image  of  the  lamp-flame  in  the  field  of 


ILLUMINATION  AND  ILLUMINATING  APPARATUS    101 

view,  but  rather  the  carrying  of  the  condenser  beyond  the  focus 
to  that  point  when  the  back  lens  of  the  objective  is  most  fully 
flooded  with  light.  In  other  words,  the  best  illumination  for 
critical  work  is  secured  at  the  stage  immediately  preceding  the 
appearance  of  the  black  dots  described  in  italics  on  p.  98.  For 
this  adjustment  experience  of  the  appearance  of  the  back  lens 
of  the  objective  is  essential,  the  eyepiece  being  removed  for 
the  purpose,  and  observation  being  made  down  the  tube  of 
the  instrument.  Acquaintance  with  the  features  of  the  back 
lens  of  the  objective  is  a  most  desirable  and  really  necessary 
acquisition  for  expert  work. 

Reliance  may  be  placed  on  the  first  method  described  for 
accurate  manipulation,  and  with  fuller  experience  the  second 
arrangement  can  be  taken  advantage  of. 

The  next  question  is,  What  amount  of  light  should  be  admitted 
from  the  condenser  in  order  to  see  the  object  at  its  best  ? 
Mr.  Nelson  has  suggested  that  the  aperture  of  the  condenser 
should  be  about  three-quarters  that  of  the  objective,  and  in  order 
to  arrange  this  it  will  be  necessary  to  remove  the  eyepiece  from 
the  microscope  and  look  down  the  tube  at  the  back  of  the  object- 
glass,  opening  the  diaphragm  of  the  condenser  to  its  fullest 
extent.  Bearing  in  mind  the  size  of  the  circle  of  light  seen, 
gradually  diminish  the  opening  of  the  diaphragm  of  the  con- 
denser until  one-quarter  of  the  back  lens  of  the  objective  is  shut 
out ;  again  put  in  the  eyepiece,  and  the  desired  amount  of 
illumination  is  arranged.  The  aperture  employed  should  be 
varied  slightly  according  to  the  transparency  or  opacity  of  the 
object  under  view. 

When  the  condenser  is  centred  and  focussed,  and  the  back 
lens  of  the  objective  is  three-quarters  filled  with  light,  a  critical 
image  is  obtained — that  is,  the  objective  is  understood  then  to 
produce  the  finest  results  it  is  capable  of. 

Mr.  Nelson's  f-cone  method  of  illumination  has  been  almost 
universally  accepted  as  a  most  practical  one ;  but  the  following 
plan,  which  was  suggested  to  the  writer  by  a  microscopical 
friend,  has  given  very  satisfactory  results.  On  examining  the 
back  lens  of  the  objective  with  a  striated  object,  such  as  Pleuro- 
sigma  angulatum,  resolved  and  focussed  upon  the  stage,  there 
will  be  seen  a  central  disc  surrounded  by  six  diffraction  spectra 


102  MODERN  MICROSCOPY 

similar  to  Fig.  41.  With  the  f-cone  illumination,  the  surround- 
ing spectra  will,  in  some  cases,  appear  to  overlap  the  central 
disc,  and  in  others  will  not  appear  to  touch  it.  Our  plan  is  to 
open  the  diaphragm  of  the  condenser  only  to  such  an  extent 
that  the  spectra  just  touch,  but  do  not  overlap,  the  central  disc. 
This  would  necessitate  that  in  some  instances  we  should  employ 
rather  less  than  a  f-cone  illumination,  and  in  others  rather 
more  than  a  f-cone.  We  have  not  been  able  to  observe  that  any 
loss  of  resolution  results  from  this  practice ;  but,  on  the  other 
hand,  in  our  opinion  detail  is  more  clearly  seen,  and  appears 
crisper  under  these  circumstances  of  illumination  than  any 
other.  This  system  is  especially  advantageous 
when  monochromatic  light  is  used. 

As   previously  mentioned,  it  will   be  found  of 
great  advantage  to  become  acquainted  with  the 
appearance  of  the  back  lens  of  the  objective  when 
Fig. 41.— Back    working;    many    hints    of    importance    may    be 
Lens  of  Ob-     nrieane(j    from    it,    enabling   manipulation    to   be 

JECTIVE.  &  '  .     . 

effected  with  increased  precision.  For  this  pur- 
pose the  '  blank '  eyepiece  referred  to  on  p.  87  is  a  most  useful 
adjunct. 

When  working  with  monochromatic  light,  the  condenser  must 
be  focussed  so  that  the  whole  of  the  light  which  is  visible  on  the 
back  lens  of  the  objective  when  the  eyepiece  is  removed  shall 
appear  as  nearly  as  possible  of  the  same  colour. 

Condensers  having  a  numerical  aperture  of  1*0  and  over 
require  to  be  immersed  in  order  that  they  may  work  at  their  full 
aperture — that  is,  a  drop  of  immersion  oil  or  Canada  balsam 
must  be  placed  between  the  top  lens  of  the  condenser  and  the 
object.  It  will  be  found  generally  that  the  condenser  is  a  little 
too  long  in  focus  for  continuity  between  the  top  lens  of  the 
condenser  and  the  under  side  of  the  object  to  be  maintained. 
Under  such  circumstances  an  additional  thin  3x1  inch  slip, 
or  a  piece  of  cover-glass,  should  be  placed  under  the  object, 
which  will  enable  the  oil  contact  to  be  maintained.  The 
distance  between  the  condenser  and  the  object  will  vary  accord- 
ing to  the  thickness  of  the  slip  on  which  the  object  is  mounted, 
and  the  intermediate  contact-glass  will  have  to  be  selected 
accordingly.      To  use  an  oil  immersion   condenser  effectively, 


ILLUMINATION  AND  ILLUMINATING  APPARATUS    103 

the  object   must   be  mounted  in  some  medium,  and  not  dry 
upon  the  cover-glass. 

As  before  mentioned,  -with  the  majority  of  condensers  stops  are 
supplied  having  the  centres  blocked  out,  as  shown  in  Fig.  42, 
by  means  of  which  dark-ground  and  oblique  illuminations  are 
obtained.  Dark-ground  illumination  gives  a  most  beautiful 
effect  to  very  transparent  objects,  such  as  infusoria,  pond-life 
specimens,  etc.  In  the  form  of  carrier  for  condensers  shown  on 
p.  95,  a  cell  is  provided  just  above  the  iris  diaphragm  to  carry 
the  stops.  One  similar  to  a  (Fig.  42)  is  placed  in  the  cell,  the 
iris  diaphragm  is  opened  completely,  the  condenser  having  been 
previously  adjusted  in  the  usual  way,  when  it  will  be  found  that 
the  oyject  will  be  illuminated,  but  the  ground  on  which  it  is  seen 
will  le  black.     Different  objectives  require  stops  of  special  sizes, 


a 


Fig.   42. — Stops  foti  Condexsers. 

which  may  be  readily  made  of  blackened  cardboard,  cut  to  the 
most  suitable  size  for  working  with  the  objective.  An  expanding 
iris  stop,  constructed  in  a  similar  manner  to  an  iris  diaphragm, 
by  means  of  which  a  variable  size  of  central  black-patch  stop  is 
secured,  is  obtainable  to  fit  most  condensers,  and  will  be  found 
of  great  utility  in  black-ground  illumination. 

These  stops  can  be  further  used  for  strengthening  the  contrast 
in  the  image  with  large  cones  of  illumination  and  objectives 
having  high  apertures.  This  method  does  not  minimize  in  any 
way  the  effective  working  of  the  objective,  for  with  objectives  of 
large  aperture  rays  may  be  present  which  only  impart  brightness 
to  the  field,  but  do  not  contribute  to  making  visible  the  fine 
detail  upon  the  object.  If  less  than  half  of  the  lateral  spectra,  as 
shown  in  Fig.  41,  are  seen  on  looking  down  the  tube  at  the  back 
lens  of  the  object-glass  with  a  striated  object  in  focus,  then  the 
central  portion  of  the  direct  beam  or  central  disc  has  no  lateral 
image  corresponding  to  it  in  the  portions  of  the  spectra  that  are 
visible.     Under  these  circumstances  that  central  portion  of  the 


104  MODEEN  MICROSCOPY 

central  disc  in  no  degree  contributes  in  enabling  the  detail  to  be 
seen,  but  only  produces  a  haze ;  by  blocking  it  out  the  haze  is 
removed,  and  there  is  a  great  improvement  in  the  resulting 
definition.     This  produces  oblique  illumination  in  all  azimuths. 

The  Choice  of  a  Condenses. — That  the  condenser  is  an 
absolute  necessity  cannot  be  too  strongly  impressed.  No  good 
results  can  be  obtained  without  it. 

Condensers,  like  objectives,  not  only  vary  in  aperture,  but  also 
in  power,  and  the  higher  the  power  of  the  condenser  the 
smaller  will  be  the  image  of  the  lamp-flame  that  it  transmits. 
Consequently,  if  a  condenser  of  high  power  is  used  with  a  low- 
power  objective,  the  illuminated  portion  of  the  field  will  be 
exceedingly  small,  while  if  a  low-power  condenser  is  used  with 
a  high-power  objective,  the  image  of  the  lamp-flame  is  so 
magnified  that  the  whole  field  is  bright,  and  it  is  not  easy  to  tell 
when  the  condenser  is  exactly  focussed.  Furthermore,  under 
such  circumstances  as  the  latter,  it  is  impossible  to  get  the  best 
effect  with  the  objective.  It  has  usually  been  recommended  that 
two  condensers,  one  of  high  and  the  other  of  low  power,  shouil 
be  included  in  a  complete  equipment,  but  the  new  types  of  con- 
densers previously  referred  to  cover  such  a  large  amount  oi 
ground  that  the  average  microscopist  really  requires  only  one. 
Choose  a  condenser  that  gives  an  aplanatic  cone  of  O90  N.A., 
and  if  the  major  portion  of  the  work  is  to  be  with  objectives  of 
low  and  medium  magnifications,  one  of  low  power  should  be 
selected ;  if  principally  with  high  powers,  a  corresponding  high- 
power  condenser  will  be  necessary.  It  must  be  remembered 
that  by  removing  the  uppermost  lens  by  unscrewing  its  cell,  the 
remaining  combinations  of  a  high-power  condenser  form  a  very 
serviceable  low-power  condenser. 

Oil  immersion  condensers  are  of  especial  utility  where  objec- 
tives of  the  largest  aperture  are  employed,  and  these,  again,  in 
several  instances  work  well  as  dry  condensers,  and  with  the 
top  lens  removed  become  low-power  condensers  of  moderate 
aperture. 

If  the  advice  given  be  followed  and  the  condenser  used 
intelligently,  reliance  will  be  placed  in  the  work  performed,  and 
far  superior  results  secured  than  would  be  possible  with  a  badly 
corrected  chromatic  condenser. 


ILLUMINATION  AND  ILLUMINATING  APPARATUS    105 


Dark-Ground  Illumination. 

Pieference  has  been  already  made  to  the  Ultra-Microscope,  and 
under  this  heading  it  will  be  necessary  to  describe  the  recently 
introduced  contrivances  for  the  obtaining  of  dark-ground  illumina- 
tion with  high-power  objectives,  and  effected  in  such  a  way  as  to 
render  visible  subjects  which  could  not  be  observed  by  other 
means.  The  effect  is  produced  with  all  the  systems  that  are  em- 
ployed, of  illuminating  the  object  with  rays  more  oblique  than  any 
the  objective  can  receive — that  is,  no  direct  light  enters  the  micro- 
scope, and  the  field  is  therefore  dark.  The  diagram  (Fig.  43) 
will  give  a  general  idea  of  the  manner  in  which  this  takes 
place.  Although  the  oblique  light  rays  do  not  enter  the  objective, 
they  cross  the  field  in  which  the  objects  lie,  and,  striking  the 
objects,  the  light  is  reflected,  refracted,  and  diffracted.  This  light 
entering  the  objective  causes  the  objects  to  stand  out  brightly  on 
a  dark  background  in  the  same  way  as  when  a  ray  of  sunlight 
enters  a  dark  room  and  renders  visible  the  extremely  minute 
dust  particles.  Thus  objects  of  excessive  minuteness  which 
would  be  invisible  by  ordinary  methods  of  illumination  are 
clearly  perceived. 

These  illuminators  have  been  found  of  great  value  in  the 
examination  especially  of  bacteria  in  the  living  state,  enabling 
identification  to  be  at  once  made,  and  rendering  unnecessary 
the  tedious  preparation  of  preparing,  staining,  etc.,  which  would 
be  involved  for  ordinary  observation  by  the  usual  methods.  In 
consequence  of  the  similarity  of  results  between  this  and  the 
ultra-microscope,  the  dark-ground  illuminators  have  been  called 
by  the  same  name ;  this,  however,  is  inaccurate,  and  the  dark- 
ground  illuminator  must  be  regarded  separately  and  distinctly. 
Exact  instruction  for  the  use  of  these  illuminators  is  given  by 
the  various  makers,  and  they  should  be  studiously  followed ;  but 
there  'are  general  principles  applicable  to  all  which  may  with 
advantage  be  indicated. 

The  illuminating  rays  have  the  numerical  aperture  extending 
from  1*0  to  1*45.  The  objectives  employed  must,  therefore,  have 
a  numerical  aperture  less  than  l'O,  otherwise  they  could  receive 
the  direct  rays,  and  dark-ground  illumination  would  no  longer  be 


106 


MODERN  MICROSCOPY 


wrTTtjTf/m  f  i.. 


obtained.  Obviously  immersion  oil  must  be  placed  between  the 
illuminator  and  the  object-slip,  or  the  aperture  in  excess  of  1*0, 
which  is  the  maximum  of  air,  will  not  be  obtainable.  It  follows 
also  that  the  object  itself  must  not  be  dry-mounted,  but  con- 
tained in  water  or  oil,  which  it  is  important  should  be  perfectly 
clean  and  clear.  This  layer  of  medium,  as  well  as  the  object 
itself,  must  be  as  thin  as  possible  ;  further,  the  object-slip  must 
be  about  the  thickness  prescribed  for  each  illuminator,  and 
must  be  free  from  scratches  and  other  defects.  The  illuminator 
must  be  accurately  centred  to  the  objective,  the  method  given  on 

p.  99,  by  placing  a  spot  of  ink  on  the 
top  surface  of  the  illuminator  for  the 
purpose,  being  probably  the  simplest 
one.  The  more  nearly  the  objective 
approaches  to  1*0  N.A..,  the  more  fully 
are  the  best  effects  realized. 

If  it  is  desired  to  use  the  higher 
magnifications  given  with  oil  immer- 
sion objectives,  such  objectives  must 
have  an  appropriate  stop  inserted  from 
the  back  to  reduce  the  numerical 
aperture  to  1*0.  The  effect  is  enhanced 
by  the  use  of  a  brilliant  source  of 
light  and  a  well-corrected  bull's-eye ; 
but  even  an  oil  lamp  with  a  bull's-eye 
will  enable  effects  to  be  obtained  and 
work  to  be  done. 
The  illuminators  of  paraboloidal  construction  as  introduced 
by  Carl  Zeiss,  and  made  in  a  modified  form  by  Watson  and 
Sons  and  R.  and  J.  Beck,  possess  great  advantage  over  ordinary 
refracting  condensers  and  dark -ground  illuminators  of  more 
complicated  construction,  inasmuch  as  the  iris  diaphragm  cuts 
out  the  rays  of  low  numerical  aperture  first,  for  as  the  diagram 
(Fig.  43)  shows,  the  paraboloid  may  be  said  to  turn  the  incident 
bundle  of  light  inside  out.  In  other  words,  the  rays  passing 
through  the  outermost  zone  of  the  iris  diaphragm  are  those  of 
lowest  numerical  aperture,  whilst  the  most  oblique  and  most 
valuable  rays  are  those  entering  near  the  centre  of  the  paraboloid. 
The  result  is  that  the  amount  of  light  can  be  reduced  without 


Fig.  43. — Diagram  showing 
Rays  passing  through 
Immersion  Paraboloid. 

A,  Front  lens  of  objective ; 
B,  rays  of  lowest  numerical 
aperture. 


ILLUMINATION  AND  ILLUMINATING  APPARATUS    107 

losing  those  invaluable  rays  of  maximum  obliquity.  These 
illuminators  are  not  suited  for  objectives  which  have  a  lower 
numerical  aperture  than  0*48. 

It  may  be  asked  why  equally  effective  results  cannot  be  obtained 
with  the  ordinary  sub-stage  condensers  and  dark-ground  stops. 
The  paraboloid  gives  a  blacker  background  than  is  possible  with 
a  condenser,  because  the  numerous  polished  surfaces  of  the  latter 
reflect  light  upon  each,  a  proportion  of  which  finds  its  way  into 
the  central  part  of  the  cone  of  rays  produced  by  the  condenser, 
and  impairs  the  blackness  of  the  field.  The  immersion  para- 
boloid must  not  be  confounded  with  the  form  of  paraboloid  used 
many  years  ago  for  dark-ground  illumination. 

The  Spot  Lens. — Before  the  sub-stage  condenser  came  into 
general  use  the  spot  lens  was  largely  utilized  for  obtaining  dark- 
ground  illumination.  It  has,  however,  been  to  a  considerable 
extent  superseded,  owing  to  the  perfection  in  which  the  same 
effect  can  now  be  obtained  with  the  condenser.  It  is  intended 
for  low  powers  only  up  to  J  inch. 

For  one  class  of  work  the  spot  lens  is  especially  advantageous. 
Most  sub-stage  condensers  have  a  very  short  focus,  and  if  organ- 
isms in  water  in  a  trough  are  being  examined,  it  is  impossible 
to  focus  the  condenser  accurately  through  the  trough  and  its 
contents.  A  spot  lens  has  a  longer  focus,  and  gives  under  these 
circumstances  the  best  results.  With  it  a  plane  mirror  and  the 
flat  of  the  wick  of  the  lamp  should  be  used  ;  the  sub-stage  that 
carries  it  should  be  moved  up  and  down  until  a  perfectly  black 
ground  is  obtained.  If  additional  brilliancy  is  required  on  the 
object,  a  stand  condenser  interposed  between  the  lamp  and  the 
mirror,  with  the  convex  side  of  the  condenser  towards  the  mirror, 
will  give  a  brighter  effect. 

The  Indian  Ink  Method. — There  is  a  recently  discovered 
method  of  showing  living  bacteria  with  a  black  background 
which  anyone  may  make  use  of  without  extra  apparatus  beyond 
a  supply  of  suitable  Indian  ink.  Burri  first  used  this  method 
in  order  to  obtain  an  absolutely  pure  culture,  as  the  bacilli  are 
1  stained '  without  being  killed,  and  the  growth  of  a  single 
bacillus  can  be  observed  under  the  microscope.  Bacilli  and 
leucocytes,  as  well  as  spirochetes,  show  up  as  clear  spaces.  The 
following  is  the  technique  recommended  by  Fruhwald,  of  Munich : 


108 


MODEEN  MICEOSCOPY 


The  surface  of  the  suspected  lesion  is  shaved  off  with  a  scalpel 
until  a  drop  of  serum  is  obtained  which  is  not  too  darkly  coloured 
with  blood.  A  loop  of  this  is  mixed,  upon  a  microscopic  slide, 
with  a  drop  of  commercial  Indian  ink.  The  mixture  is  then 
spread  with  the  edge  of  a  coverslip.  The  film  dries  within  the 
minute,  and  it  can  be  examined  immediately  with  the  oil  immer- 
sion lens.  The  spirochetes  are  seen  as  bright  spirals  on  a  dark 
brown  field.  Leucocytes,  bacteria,  and  other  spirochetes  are 
also  seen  as  clear  spaces.  The  effect  produced  is  dependent  on 
the  fact  that  the  particles  of  Indian  ink  are  smaller  than  the 
bacteria,  and  hence,  when  a  mixture  of  the  two  is  allowed  to 
settle  on  a  slide,  the  bacteria  remain  as  clear  transparent  spaces 
on  a  dark  ground. 


The  Polariscope.* 

This  consists  of  two  parts,  each  composed  of  a  Nicol  prism 
of  Iceland  spar  in  a  suitable  mounting — one  called  the  polarizer, 


Fig.  44. — Polarizer. 


Fig.  45. — Analyzer. 


which  fits  into  the  sub-stage,  and  the  other  the  analyzer,  which 
is  usually  inserted  between  the  nosepiece  of  the  microscope  and 
the  objective.  By  its  means  light  is  split  up  into  its  component 
parts,  and  most  beautiful  colour  effects  are  obtained.  The 
polarizer  has  a  flange  beneath,  by  which  it  can  be  rotated,  and 
in  this  way  the  colours  are  varied.  In  examining  certain  chemical 
crystals,  geological  slides,  etc.,  it  brings  into  view  structure  which 
without  it  would  hardly  be  detected,  and  for  this  it  is  largely 

*  Further  instructive  information  regarding  the  polariscope  is  given  in  the 
treatment  of  Petrological  Microscopes  on  p.  219. 


ILLUMINATION  AND  ILLUMINATING  APPARATUS    109 

used  in  analytical  work.  In  some  instruments  the  analyzer 
prism  is  fitted  in  the  body.  This  is  rather  an  inconvenience, 
unless  the  instrument  be  designed  especially  for  petrology.  For 
a  binocular  microscope,  however,  if  it  is  placed  between  the 
nosepiece  and  the  objective,  it  causes  a  separation  between  these 
two,  which  interferes  with  the  performance  of  the  binocular 
prism,  because  the  closer  the  back  lens  of  the  objective  can  be 
brought  to  the  binocular  prism,  the  more  perfect  will  the  vision 
be.  Under  these  circumstances  the  monocular  tube  only  is 
generally  used  ;  or  the  analyzer  prism  can  be  mounted  over  the 
top  of  the  eyepiece  of  the  monocular  tube.  The  theory  of 
polarization  is  fully  explained  in  connection  with  Penological 


Fig.  46. — Darker's  Selenites. 

Microscopes  on  p.  219.  For  use  with  the  polariscope,  varieties 
of  tints  and  a  background  of  colour  can  be  obtained  by  the 
employment  of  selenite  films.  These,  in  the  cheapest  form, 
are  mounted  in  the  same  way  as  ordinary  microscopic  objects ; 
but  a  still  greater  variety  of  effect  can  be  obtained  by  having 
selenites  fitting  into  a  carrier  to  come  between  the  polarizer 
and  the  stage  in  a  sub-stage  microscope.  We  illustrate  one 
(Fig.  46)  by  E.  and  J.  Beck.  In  this  form  each  of  the  selenites 
is  provided  with  a  ring  which  rotates.  The  three  being  one 
over  the  other,  either  two  or  all  three  can  be  rotated  together 
or  in  opposite  directions  to  one  another,  and  the  effect  is  most 
striking.  An  inexpensive  modification  of  this  is  made  by  Swift 
and  Sons  and  others,  called  the  mica-selenite  stage,  as  shown  in 
Fig.  47.  This  consists  of  a  film  of  mica  made  to  rotate  in  a 
brass  plate,  upon  which  the  object  is  laid,  and  beneath  it  is  a 


110 


MODEEN  MICROSCOPY 


carrier  with  three  separate  selenites.  These  can  each  be  pushed 
separately  beneath  the  mica  and  the  latter  rotated.  By  this 
means  all  the  different  tints  obtainable  with  any  number  of 
selenite  films  can  be  produced.     It  can  be  employed   on  any 


Fig.  47. — Mica-Selexite  Stage. 


microscope.  To  get  greater  brilliancy  the  polarizer  can  be  made 
to  fit  into  the  sub-stage  condenser  on  the  under  side,  and  the 
Abbe  chromatic  and  achromatic  condensers  referred  to  previously 
are  particularly  suitable  for  this  arrangement. 


The  Bull's-Eye  Condenser. 

Many  objects,  being  opaque,  cannot  be  viewed  by  light  from 
beneath,  and  consequently  have  to  be  illuminated  from  above. 
In  order  to  do  this  a  bull's-eye  condenser  is  necessary.  This 
usually  consists  of  a  plano-convex  lens  mounted  on  a  stand,  as 
shown  in  Fig.  48.  This  has  a  ball  and  socket  joint  and  a 
sliding  telescopic  tube,  by  means  of  which  the  lens  can  be  placed 
in  any  desired  position.  Both  Mr.  Nelson  and  Mr.  Conrady  have 
introduced  improvements  in  the  construction  of  bull's-eye  con- 
densers in  order  to  reduce  the  large  amount  of  spherical  aberra- 
tion which  is  a  necessary  accompaniment  of  the  single  lens. 
These  improved  forms  consist  of  either  two  or  three  lenses 
in  combination,  and  the  advantage  obtained,  especially  in 
photography,  is  well  worth  the  additional  outlay. 

The  Use  of  the  Bull's-Eye  Condenser. — The  diagram  on 
p.  112  will  show  that  effects  are  produced  by  the  bull's-eye 
according  to  the  position  the  light  is  placed  in  relation  to  it. 
For  the  illumination  of  opaque  objects  on  the  stage  of  the  micro- 
scope, a  brilliant  point  of  light  is  required.     Fig.  49,  B,  shows  that 


ILLUMINATION  AND  ILLUMINATING  APPARATUS    111 

to  secure  this  the  bull's-eye  should  be  set  midway  between  the 
illunrinant  and  the  object.  It  will  be  obvious  that  the  light  and 
bull's-eye  must  be  in  alignment  with  the  object,  and  it  will  be 
generally  found  more  convenient  if  the  light  be  set  somewhat 
above  the  level  of  the  stage,  so  that  the  shadows  produced  by  the 


Fig.   48.— Stand  Condenser. 


object  may  not  be  too  pronounced.  For  certain  work,  however, 
it  is  necessary  to  obtain  a  parallel  beam  with  a  bull's-eye  ;  to  do 
this  the  light  must  be  set  in  the  principal  focus  of  the  bull's-eye, 
as  shown  in  Fig.  49,  C.  This  arrangement  will  often  be  found 
useful  for  illuminating  objects  viewed  with  low  powers,  from 
beneath  the  stage,  when  no  sub-stage  condenser  is  employed  ; 
and  for  low-power  photography  this  is  particularly  convenient. 
If  now  the  light  be  placed  some  distance  from  the  bull's-eye, 


112 


MODERN  MICROSCOPY 


as  in  Fig.  49,  A,  the  focal  point  will  be  seen  to  be  shorter.  The 
effective  working  of  the  bull's-eye  can  only  be  obtained  by  know- 
ledge of  these  rules  and  practice.  It  will  be  found  advantageous 
to  conduct  some  experiments,  holding  a  piece  of  card  in  the  path 
of  the  rays,  and  observing  the  effects. 

It  must  be  clearly  stated  that  no  matter  what  lens  is  used,  no 


_^I 


n 


s 


L  — 


F 


"X 


S 


F 


c. 


Fig.  49. — Diagrams  showing  the   Effect  of  placing  the  Bull's-Eye  Con- 
denser at  Different  Distances  from  the  Source  of  Light. 

increase  of  light  is  obtained  in  excess  of  that  of  the  illuminant ; 
all  that  it  does  is  to  collect  and  concentrate  that  light  ;  and 
Mr.  A.  E.  Conrady,  in  connection  with  his  Watson-Conrady 
achromatized  bull's-eye,  has  pointed  out  that,  although  not  of 
large  diameter  (2  J  inches),  it  utilizes  all  the  rays  from  the  illumi- 
nant which  it  is  possible  to  employ  for  microscopical  purposes. 


ILLUMINATION  AND  ILLUMINATING  APPARATUS    113 

The  bull's-eye  condenser  is  often  used  in  conjunction  with  the 
sub-stage  condenser,  to  enable  the  field  to  be  evenly  illuminated 
with  the  latter,  as  mentioned  on  p.  100. 


The  Parabolic,  or  Side  Silver  Reflector. 

To  enhance  the  effects  obtained  with  opaque  objects  with  stand 
condensers,  the  side  silver  reflector  is  most  advantageous.  The 
arm  on  which  this  is  mounted  is  attached  to  either  the  stage  or 
limb  of  the  microscope,  or  fitted  between  the  nosepiece  and  the 
objective.  The  reflector  consists  of  a  highly-polished  silver  para- 
bolic speculum.  This  reflector  is  placed  by  the  side  of  the  object, 
and  light  is  directed  from  the  lamp  through  the  bull's-eye  on 
to  its  centre,  and  then  thrown  by  the  reflector  on  to  the  object. 
Most  brilliant  opaque  illumination  may  be  obtained  by  this 
means. 

The  Vertical  Illuminator. 

The  merits  of  this  piece  of  apparatus  were  but  little  appre- 
ciated until  the  study  of  metal  surfaces  microscopically  was 
taken  in  hand.  Its  use  had  been  almost  exclusively  restricted  to 
ascertaining  whether  the  specimens  were  mounted  in  contact 
with  the  under  surface  of  the  cover-glass — information  which  is 
of  importance  when  an  oil  immersion  object  is  being  employed 
on  an  object  mounted  dry,  for  if  the  specimen  is  not  adherent  to 
the  cover-glass  it  cannot  be  seen  at  all  with  this  illuminator.  It 
is  also  used  for  the  resolution  of  the  markings  on  diatoms  that 
are  mounted  dry  on  the  cover-glass  ;  in  this  case  it  is,  however, 
only  of  value  with  immersion  objectives. 

In  metallurgical  work  it  has  rendered  permissible  the  examina- 
tion of  surfaces  of  metals  with  high-power  objectives.  The  surface 
to  be  examined  is  highly  polished  and  then  etched  with  acid, 
liquorice  juice,  or  other  medium,  as  will  be  found  described  on 
p.  196,  and  no  covering-glass  at  all  is  used.  It  is  made  in  two 
patterns,  one  with  a  prism,  and  the  other  with  a  disc  of  cover - 
glass.  Metallurgists  usually  have  the  two  forms,  finding  one 
more  serviceable  than  the  other  under  certain  conditions  and 
with  different  specimens.     They  both  have  their  distinct  value. 

8 


114  MODERN  MICROSCOPY 

Generally  speaking,  for  this  class  of  work  the  prism  pattern  is  the 
better  for  low-power  objectives,  and  the  disc  form  for  the  high 
powers.  The  latter  is  illustrated  in  Fig.  50,  and  may  be  either 
built  in  the  body  of  the  microscope  or  screwed  to  the  nosepiece 
of  the  microscope  above  the  objective  ;  the  one  illustrated  is 
arranged  for  the  latter.  The  disc  of  cover-glass  attached  to  a 
little  clip  having  a  milled  head,  a,  is  placed  through  a  slot  in  the 
fitting,  b,  and  set  at  an  angle  of  45°  to  the  optic  axis.  Light  is 
received  on  this  disc  of  glass  through  the  small  opening  in  the 
body  of  the  fitting,  b,  and  it  is  totally  reflected  through  the  object- 
glass  on  the  object,  the  objective  acting  as  its  own  achromatic 
condenser.  In  order  that  the  light  may  be  focussed  on  the  object, 
the  lamp-wick  from  which  the  light  is  being  obtained  must 
be  the  same  distance  from  the  reflector  as  the  latter  is  from 
the  diaphragm  of  the  eyepiece,  if  a  positive   eyepiece  is  being 

ct  b        fiSV;s) 


Fig.  50. — Vertical  or  Disc  Illuminator. 

used,  or  to  the  eye-lens,  if  a  Huyghenian  or  negative  eyepiece 
is  employed. 

To  save  a  frequent  readjustment,  some  workers  have  the  lamp 
fitted  with  a  projecting  tube,  which,  when  in  contact  with  the 
side  of  the  microscope,  gives  the  exact  distance  at  which  it 
should  be  set  in  relation  to  the  vertical  illuminator. 

To  know  what  can  be  done  with  the  vertical  illuminator,  it  is 
necessary  that  a  fitting  be  attached  to  it  into  which  stops  and 
diaphragms  can  be  placed,  and  for  the  effective  working  a  bull's- 
eye  condenser  is  set  in  such  relation  to  the  lamp  that  the  aerial 
image  of  the  lamp  is  produced  at  a  distance  from  the  vertical 
illuminator  equal  to  that  from  the  vertical  illuminator  to  the 
diaphragm  or  eye-lens  of  the  eyepiece,  according  to  the  eyepiece 
that  is  in  use.  The  position  of  the  aerial  image  must  be  ascer- 
tained by  the  use  of  a  white  card  screen  ;  by  this  means  the 
same  effect  will  be  produced  as  when  a  stop  or  diaphragm  is 
placed  immediately  over  the  vertical  illuminator. 


ILLUMINATION  AND  ILLUMINATING  APPARATUS     115 

Glare  with  the  Vertical  Illuminator. — Metallurgists  are 
frequently  troubled  with  glare  which  is  constantly  present  in 
the  field  of  view  when  using  a  vertical  illuminator,  and  it  is 
often  so  obtrusive  as  to  seriously  diminish  sharpness  of  defini- 
tion and  perception  of  detail.  This  is  obviated  to  a  great  extent 
by  having  the  objectives  set  in  special  mounting,  so  that  the 
back  lens  approaches  as  nearly  as  possible  to  the  reflector  of  the 
vertical  illuminator ;  but  it  may  be  modified — in  fact,  for  prac- 
tical purposes,  eliminated — by  using  an  iris  diaphragm  quite 
close  to  the  source  of  light  or  in  front  of  the  bull's-eye,  and 
reducing  the  aperture  through  which  the  light  passes  to  such  a 
point  as  will  remove  the  objectionable  glare.  This  will  not 
affect  the  brilliance  of  the  actual  image,  for  only  the  precise 
amount  of  light  which  can  be  utilized,  no  less  and  no  more,  will 
pass  through  the  objective. 


CHAPTEE  IV 

ACCESSOEY  APPAKATUS 

The  Lamp. 

It  is  most  important  that  the  microscopist  should  possess  a  good 
and  suitable  lamp,  otherwise  he  cannot  work  to  the  greatest 
advantage.  The  amateur  will  often  be  found  working  with  a 
reading-lamp  or  an  ordinary  oil-lamp,  but  good  work  can  never 
be  done  conveniently  by  this  means.  There  are  two  or  three 
important  points  which  must  be  borne  in  mind.  In  the  first 
place,  if  light  is  proceeding  from  the  one  illuminating  point 
only,  and  the  remainder  of  the  room  is  dark  while  using  the 
microscope,  a  great  deal  better  effect  can  be  produced  than  if 
the  whole  room  be  illuminated.  In  the  next  place,  a  small 
brilliant  source  of  light  is  far  better  than  a  large  one.  In  recent 
years  special  attention  has  been  paid  to  this  matter,  with  the 
result  that  lamps  have  been  constructed  with  which  the  best 
work  may  be  accomplished.  The  following  are  desirable  features 
which  should  be  embraced  by  a  good  microscope  lamp :  The 
reservoir  for  oil  should  be  large  in  diameter  and  flat,  so  that  the 
light  may  be  brought  down  very  close  to  the  table.  For  this 
reservoir  glass  is  usually  preferable  to  metal,  it  being  much 
cleaner,  and  the  worker  is  able  to  tell  when  his  oil  is  getting 
exhausted  ;  whereas  with  a  metal  reservoir,  unless  careful  reckon- 
ing is  kept,  in  the  middle  of  some  important  observation  the 
light  may  go  out  from  want  of  oil.  A  J-inch  wick  is  generally 
found  to  be  sufficient.  We  strongly  deprecate  the  use  of  glass 
chimneys.  They  are  always  liable  to  get  broken  very  easily,  and 
become  a  source  of  expense,  in  addition  to  which,  if  away  from 
town,  there  is  a  possibility  of  not  being  able  to  get  the  right 
kind,  and  so  work  may  be  delayed.    Far  better  will  be  found  the 

116 


ACCESSOEY  APPARATUS 


117 


metal  chimneys  now  made  by  nearly  all  opticians,  with  a  carrier 
for  a  3  x  1  or  3  x  1J  inch  slip.  It  is  obvious  that  if  the  slip  be 
broken  it  can  be  immediately  replaced,  it  being  part  of  the 
microscopist's  average  stock.  It  is  also  desirable  that  the  bar 
on  which  the  lamp  is  raised  and  lowered  on  the  stand  should  be 
a  square  one.  If  round 
in  shape,  the  lamp  is  apt 
to  swing  round  on  the 
stand  and  the  whole  to 
topple  over.  This  is  an 
impossibility  with  the 
square  bar.  Such  a  lamp 
is  shown  in  Fig.  51,  by 
Swift  and  Son,  and  modi- 
fications of  it  can  be  ob- 
tained from  most  dealers. 

A  similar  model  of 
lamp  which  has  rack- 
work  and  sliding  adjust- 
ments in  the  vertical 
direction,  and  a  quick 
acting  screw  for  lateral 
adjustments,  is  also  ob- 
tainable. Its  mechanical 
arrangements  enable  the 
light  from  the  lamp  to 
be  shifted  just  the  slight 
amount  that  is  often 
needed,  and  obviate  the 
necessity  for  moving  the  lamp  itself,  as  would  otherwise  have  to 
be  done. 

Electric  Lamps.  —  The  increasing  use  of  electric  light  has 
caused  some  attention  to  be  given  to  this  form  of  illuminant 
for  microscopical  work.  There  are  several  very  handy  patterns 
that  can  be  connected  to  the  ordinary  house  circuit.  It  is  not 
suited  for  critical  work,  but  the  average  observer  has  so  much 
to  do  for  which  such  a  lamp  is  suitable  and  immediately  available 
that  it  is  becoming  largely  used.  The  lamp  should  have  a  frosted 
bulb,  and  the  stand  should  permit  of  universal  movements. 


Fig.  51. 


-Microscope  Lamp  with  Metal 
Chimney. 


118  MODERN  MICROSCOPY 

The  Nernst  electric  lamp  has  been  found  a  brilliant  and  useful 
source  of  light.  The  narrow  filament  is  a  cause  of  inconvenience, 
but  its  utility  is  such  that  it  is  the  subject  of  constant  improve- 
ments and  experiment.  The  opticians'  catalogues  should  be 
referred  to  for  latest  developments. 

Incandescent  Gas  Lamps.  —  A  very  brilliant  and  efficient 
illumination  can  be  obtained  from  an  incandescent  burner  on 
a  table-stand  of  suitable  height  properly  shaded.  The  reticula- 
tions of  the  mantle  would  render  it  objectionable,  but  this  can 
be  obviated  by  placing  an  iris  diaphragm  a  short  distance  from 
the  light  in  the  same  manner  as  is  described  for  the  lamp  above, 
or  a  narrow  slit  would  answer  the  purpose  equally  as  well ;  the 
diaphragm  or  slit  would  then  be  focussed  by  the  sub-stage 
condenser,  and  treated  as  the  source  of  light. 

Objective  Changers. 

The  Revolving  Nosepiece. — Time-saving  arrangements  will  be 
found  valuable  in  working,  and  the  nosepiece  is  one  of  these  ; 

it  is,  in  fact,  almost  a  necessity,  especially 
where  constant  change  from  a  low-  to  a 
high-power  objective  is  necessary.  It  is 
screwed  into  the  fixed  nosepiece  of  the 
microscope,  and  is  made  to  carry  either 
Fig.  52.— The  Dustproof  two,  three,  or  four  objectives,  termed  re- 
Ob0je™ves.FOR  THEEE    spectively  the  double,  triple,  or  quadruple 

nosepiece.  Each  of  the  objectives  can  in 
turn  be  rotated  into  the  optical  axis,  thus  saving  the  necessity 
of  unscrewing  an  objective  and  screwing  another  on  in  order 
to  get  a  variation  of  power.  In  hospitals,  laboratories,  etc.,  it 
is  usual  to  have  one  of  these  fitted  to  every  instrument.  The 
dustproof  pattern  has  found  special  favour,  rendering  unneces- 
sary the  removal  of  objectives  after  use.  One  of  these  is  illus- 
trated (Fig.  52).  They  are  to  be  had  made  of  an  aluminium 
alloy  which  is  extremely  light,  reducing  the  strain  on  the  body- 
tube.  Any  microscope  having  the  universal  size  of  thread  for 
objectives  will  carry  a  revolving  nosepiece  ;  no  special  adaptation 
is  required.  When  the  revolving  nosepiece  is  screwed  home,  the 
objectives  not  in  use  must  point  towards  the  middle  of  the  front 


ACCESSORY  APPARATUS 


119 


of  the  stage,  otherwise  in  rotating  the  objectives  they  are,  with 
low  powers,  apt  to  foul  the  rackwork  bar  of  the  microscope. 

Objective  Changers  by  Zeiss. — It  will  be  readily  perceived 
that  however  accurately  a  revolving  nosepiece  may  be  made,  the 
same  portion  of  the  object  will  not  occupy  the  centre  of  the 
e  a  when  different  objectives  are  brought  into  use  if  the  ob- 
jectives themselves  have  not  identical  centres.  Variations  in 
this  respect  frequently  occur,  especially  with  objectives  by  different 


a 


Fig.  53.— Sliding  Objective  Changer,  by  Zeiss. 

a,  Tube-slide  ;  b,  objective  slide  with  objective  attached  ;  c,  in  section,  tube 

slide  and  objective  slide  united. 

makers,  and  recognizing  this,  Carl  Zeiss  introduced  an  objective 
changer  consisting  of  two  parts,  one  called  the  '  tube  slide '  (a), 
which  is  screwed  into  the  nosepiece  end  of  the  microscope-tube, 
and  the  other  the  '  objective  slide'  (b),  to  which  the  objective  is 
screwed.  A  separate  objective  slide  is  required  for  each  objective. 
The  accompanying  illustrations  give  an  excellent  idea  of  the 
construction.  It  will  be  seen  that  the  two  ■  slides  '  are  united 
by  a  sliding  fitting,  the  upper  of  which  is  inclined  downwards, 
so  that  when  an  objective  is  withdrawn  it  is  lifted  from  the 


120  MODERN  MICROSCOPY 

object.  Attached  to  the  lower  slide  are  two  screws,  one  of  which 
serves  to  centre  the  object  longitudinally,  and  the  other  trans- 
versely. By  this  means  all  objectives  can  be  set  to  a  common 
centre  and  permanently  retained  so.  The  simplest  mode  of 
procedure  is  to  set  a  small  specimen  in  the  centre  of  the  field 
with  the  objective  of  highest  power,  without  either  of  the  '  slides  ' 
in  position — that  is,  the  objective  should  be  screwed  into  the 
microscope  direct.  This  is  done  so  that  the  correct  centre  may 
be  first  ascertained.  Then  remove  the  objective  and  fix  the  '  tube 
slide  '  to  the  microscope,  and  screw  the  same  objective  to  the 
1  objective  slide,'  and  if,  when  in  position,  the  object  no  longer 
occupies  the  exact  centre  of  the  field,  the  set  screws  are  used 
until  it  does  so.  The  next  highest  objective  should  be  treated  in 
the  same  way,  and  so  on  with  other  objectives.  By  retaining  each 
objective  in  its  own  '  slide,'  accuracy  is  maintained.  This  device 
is  particularly  convenient  with  microscopes  having  a  concentric 
rotating  stage,  the  object  always  remaining  in  the  field.  This 
is  a  most  excellent  objective  changer,  but  its  use  demands  care 
and  more  leisure  than  the  laboratory  worker  can  usually  bestow ; 
hence  the  revolving  nosepiece  is  far  more  generally  used. 

The  Nosepiece  Iris  Diaphragm,  or  Davis's  Shutter. 

This  is  a  very  compactly  made  iris  diaphragm,  which  is  placed 
between  the  nosepiece  of  the  microscope  and  the  objective.     Its 

special  function  is  to  enable  the 
aperture  of  an  objective  to  be  de- 
creased, so  that  it  may  be  used  with 
dark-ground  illumination,  or  to  in- 
crease penetration  when  examining 
objects  having  several  planes.  For 
photographing  opaque  objects  with 
low  powers  it  enables  the  appear- 
Fig.  54.— Davis's  Shutter.         ance  of  a  small  round  object,  such 

as  a  moth's  egg,  to  be  taken  quite 

sharply.     A  similar  result  has  been  attained  by  mounting  an  iris 

diaphragm  between  the  lens  combinations  of  low-power  objectives. 

The  Davis's  shutter  is  furthermore  very  useful  for  examining 

and  experimenting  with  the  diffraction  spectra  seen  on  looking 


ACCESSORY  APPARATUS  121 

down  the  microscope-tube  at  the  back  of  the  objective,  when  the 
eyepiece  is  removed  and  a  striated  object  is  being  examined. 

This  iris  diaphragm  must  have  its  aperture  perfectly  central 
and  the  threads  quite  true.  The  aperture  of  the  iris,  when 
completely  opened,  should  be  as  great  as  it  is  possible  for  the 
inside  of  the  mount  of  an  objective  to  be,  but  the  box  of  the  iris 
diaphragm  must  not  be  so  large  as  to  touch  the  bearings  in  which 
the  tube  of  the  microscope  is  raised  and  lowered.  It  is  well  to 
have  the  lever  of  the  iris  diaphragm  working  in  front  of  the 
microscope-tube,  and  so  that  this  may  be  easily  arranged  the 
body  of  the  iris  should  be  so  mounted  that  it  may  be  rotated, 
but  should  be  very  stiff  in  its  rotary  movement. 

Camera  Lucida. 

This  is  designed  to  assist  in  drawing  objects  seen  in  the 
microscope.  Photo-micrography  has  to  a  large  extent  super- 
seded it,  but  does  not  depict  the  several  planes  of  an  object  in 
the  realistic  manner  that  is  possible  in  a  drawing ;  still,  there 
are  a  great  many  who  prefer  this  method  to  any  other.  Dr.  Beale's 
neutral-tint  reflector,  which  is  supplied  by  all  the  opticians,  is 
the  cheapest,  and  a  very  good  form.  It  consists  essentially  of  a 
neutral- tint  glass — in  which  the  image  of  the  object  is  reflected 
— mounted  in  a  frame  to  fit  over  the  eyepiece.  The  method  of 
using  it  is  as  follows  :  The  microscope  is  set  in  a  horizontal 
position,  with  the  centre  of  the  eyepiece  10  inches  from  the 
table.  Illumination  is  arranged  in  the  ordinary  way.  The  cap 
of  the  eyepiece  is  removed,  and  the  neutral-tint  reflector  is  fitted 
in  place  of  it,  and  is  so  arranged  that  the  centres  of  the  neutral- 
tint  glass  and  the  eye-lens  of  the  eyepiece  are  in  alignment,  the 
former  being  set  at  an  angle  of  45°.  On  looking  on  this  neutral- 
tint  glass  from  the  upper  side,  a  disc  of  bright  light  will  be  seen 
on  it,  and  if  a  piece  of  white  paper  be  spread  below  on  the  table, 
on  further  examination  the  outlines  of  the  object  will  appear  to 
be  upon  the  paper.  If  a  pencil  be  now  taken,  the  specimen  can 
be  sketched  in  its  magnified  form.  This  will  be  found  somewhat 
difficult  at  first,  nearly  every  worker  seeming  to  find  it  necessary 
to  work  in  some  special  manner  of  his  own ;  but  the  secret  of 
success  is  to  arrange  the  balance  of  illumination  by  turning  the 


122 


MODERN  MICROSCOPY 


lamp-wick  up  and  down  until  a  degree  of  light  is  found  at  which 
the  pencil-point  and  image  can  be  distinctly  seen.  When  using 
students'  microscopes,  the  eyepieces  of  which  have  no  caps,  it  is 
usual  to  remove  the  eyepiece,  fit  the  tube  of  the  reflector  to  the 
outside  of  the  top  of  the  draw- tube,  then  reinsert  the  eyepiece 
and  set  the  neutral-tint  glass  in  position.  The  tinted  glass  is 
usually  mounted  on  an  arm  which  has  a  joint,  so  that  it  may  be 
turned  out  of  the  way  when  not  required  without  detaching  the 
piece  of  apparatus  from  the  microscope.    The  distance  of  10  inches 


Fig.  55. — Abbe  Camera  Lucida. 


between  the  eyepiece  and  the  table  is  maintained,  whether  the 
microscope  has  a  6  or  a  10  inch  tube-length. 

The  Beale's  neutral-tint  possesses  the  disadvantage  of  revers- 
ing the  image  that  is  seen  with  it.  Mr.  Ashe,  therefore,  devised 
a  modification  known  by  his  name,  which  overcomes  this  defect, 
while  maintaining  the  simple  principle  of  the  Beale's  pattern. 
It  was  described  in  the  Journal  of  the  Quekett  Club.  Of  more 
expensive  description,  but  considered  the  best  at  present  made, 
is  the  Abbe  camera  lucida  (Fig.  55).  The  microscope  may  be 
used  in  a  vertical  or  any  inclined  position  with  this  apparatus. 
Its  construction  and  the  manner  of  using  is  as  follows :  Mounted 
in  a  cap,  which  is  fitted  immediately  above  the  eyepiece,  are  two 
right-angle  prisms  ;  these  are  cemented  together  and  form  a  cube. 
One  of  the  cemented  surfaces  is  silvered,  but  a  small  central 
disc  is  left  clear,  through  which   the  object  is  viewed  in  the 


ACCESSORY  APPARATUS  123 

ordinary  way.  The  prisms  are  so  set  that  the  image  of  the 
paper  on  which  the  drawing  is  to  be  made,  and  which  is  reflected 
by  a  mirror  to  the  prisms,  is  by  them  conveyed  to  the  eye. 
Thus  the  pencils  of  light  reach  the  eye  coincidently  from  both 
the  microscope  and  the  paper,  and  when  drawing  the  object  the 
pencil-point  appears  in  the  field  of  view  very  distinctly,  and  the 
minutest  details  can  be  exactly  traced.  Low-power  eyepieces 
should  be  used  with  this  camera  lucida.  There  are  many  other 
forms  peculiar  to  individual  makers,  possessing  more  or  less 
merit,  some  of  which  may  be  used  with  the  tube  in  any  position, 
the  Swift-Ives  pattern,  made  by  Swift  and  Son,  being  particu- 
larly efficient ;  it  can  be  used  with  the  microscope  set  vertically 
or  inclined. 

The  Measurement  of  Objects. 

There  are  three  ways  in  which  this  may  be  effected  : 

1.  By  having  the  stage  divided — applicable  to  mechanical 

stages  only. 

2.  By  means  of  a  camera  lucida  and  a  stage  micrometer. 

3.  By  means  of  eyepiece  and  stage  micrometers. 

1.  If  the  movements  of  a  mechanical  stage  are  divided  and 
read  by  verniers  to  very  small  parts  of  an  inch  or  millimetre, 
the  measurement  of  an  object  can  be  effected  by  having  in  the 
eyepiece  a  disc  of  glass  with  a  diamond-cut  line  across  the  centre. 
The  object  that  it  is  desired  to  measure  is  set  with  one  point 
exactly  against  the  diamond-cut  line,  which,  of  course,  will  appear 
in  the  field,  and  the  readings  of  the  stage  divisions  taken.  The 
stage  is  then  slowly  moved  along  by  means  of  the  milled  head 
until  the  other  edge  of  the  specimen  to  be  measured  is  exactly 
touching  the  line.  The  readings  of  the  stage  divisions  are  again 
taken,  and  by  subtracting  one  from  the  other  the  measurement 
will  be  ascertained.  For  quick  work,  and  without  extraneous 
appliances,  this  is  fairly  accurate,  and  largely  used. 

2.  Camera  Lucida  and  Stage  Micrometer. — A  stage  micrometer 
usually  consists  of  a  number  of  lines  photographed,  or  ruled  with 
a  diamond  on  a  slip  of  glass  to  the  scale  of  yuv  or  tuVs  part 
of  an  inch,  or  the  TV  and  -x  Jg  of  a  millimetre.  This  is  put  on 
the  stage  and  focussed  like  an  ordinary  object.  The  camera 
lucida  is  then  fixed  to  the  eyepiece,  and  the  micrometer  lines 


124  MODEKN  MICEOSCOPY 

are  projected  on  to  a  piece  of  paper  in  the  same  way  as 
when  drawing  an  object  explained  on  p,  122.  The  lines  so 
projected  are  then  measured,  and  supposing  the  lines  of  the 
micrometer,  which  are  Ttro  of  an  inch  apart,  appear  when  drawn 
on  the  paper  1  inch  apart,  it  is  at  once  known  that  the  magnify- 
ing power  in  use  is  100  diameters.  The  object  may  be  measured 
in  the  same  manner.  Measurements  should  be  taken  about  the 
centre  of  the  field,  and  not  towards  the  edge,  especially  with  high 
powers,  as,  owing  to  curvature  of  the  field,  the  outer  edges  appear 
more  highly  magnified  than  the  centre. 

3.  The  Eyepiece  Micrometer  and  Stage  Micrometer.  —  The 
stage  micrometer,  as  previously  described,  is  placed  on  the  stage, 
and  a  somewhat  similar  micrometer  is  put  into  the  eyepiece. 
This  latter  is  generally  divided  into  hundredths  of  an  inch,  but 
no  exact  value  is  needful  so  long  as  the  lines  are  equidistant. 
On  focussing  the  stage  micrometer  the  two  sets  of  lines  will  appear 
in  the  field  at  once.  It  is  now  desirable  to  ascertain  how  many 
divisions  of  the  eyepiece  micrometer  are  included  between  one  of 
the  spaces — that  is,  j^  of  an  inch — of  the  stage  micrometer. 
Perhaps  it  will  be  found  that  there  will  be  several  lines  of  the 
eyepiece  micrometer  and  a  fraction  in  that  space,  and  in  order 
that  this  fraction  may  be  obviated  the  draw-tube  should  be 
slightly  pulled  out,  which  will  give,  of  course,  an  increased 
amplification,  until  a  certain  number  of  the  lines  on  the  eyepiece 
micrometer  are  exactly  equal  to  a  division  or  divisions  on  the 
stage  micrometer.  We  will  imagine  that  the  number  of  eyepiece 
micrometer  lines  that  fill  TJo  of  an  inch  of  the  stage  micro- 
meter is  five.  The  stage  micrometer  is  now  removed,  and  the 
object  to  be  measured  replaces  it.  The  lines  of  the  eyepiece 
micrometer  will  still  be  seen  in  the  field,  and  bearing  in  mind 
that  five  of  these  lines  equal  y^o  of  an  inch,  any  part  of  the  object 
can  at  once  be  measured.  It  must  be  remembered,  however, 
that  with  every  objective  and  at  every  tube-length  an  estimation 
of  the  value  of  the  eyepiece  micrometer  is  necessary. 

To  give  greater  facility  and  accuracy,  a  form  of  eyepiece 
micrometer  is  used,  devised  by  Jackson,  which  is  fitted  in  a 
frame,  and  by  means  of  a  micrometer  screw  traverses  the  object. 
If  there  be  no  mechanical  stage  to  the  instrument,  it  is  very 
difficult  to  set  a  special  part  of  the  object  against  the  micrometer 


ACCESSORY  APPAEATUS 


125 


for  measurement,  especially  with  high  powers.  This  form  of 
micrometer  surmounts  this  difficulty.  The  ordinary  eyepiece 
micrometers  necessitate  no  alteration  to  ordinary  eyepieces, 
because  they  rest  on  the  diaphragm  inside,  but  the  Jackson  form 
requires  that  the  outer  tube  of  the  eyepiece  shall  be  cut  to  receive 
the  carrier  for  the  micrometer.  Fig.  56  shows  an  eyepiece  with 
the  Jackson  micrometer,  m,  in  position. 

There  is  yet  another  form  of  eyepiece  micrometer,  called  the 
Ramsden  screw  micrometer,  which  consists  of  an  eyepiece 
containing  two  webs  or  wires,  one  fixed,  the  other  travelling  by 
means  of  a  screw  having  100  threads  to  the  inch.  The  milled 
head  of  this  screw  is  divided  into  100  parts.     Across  the  field  are 


Fig.  56. — Jackson  Micrometer 
fitted  to  Eyepiece. 


Fig.  57. — Ramsden  Screw 
Micrometer. 


very  small  equidistant  V-shaped  teeth,  the  interval  between  each 
of  which  corresponds  to  one  complete  revolution  of  the  milled 
head.  The  value  of  these  teeth  is  taken  against  the  stage 
micrometer,  and  the  object  placed  on  the  stage.  One  edge  of  the 
object  is  then  brought  against  the  fixed  wire,  and  the  travelling 
wire  moved  to  the  other  part  that  it  is  desired  to  gauge.  By 
then  counting  the  number  of  intervening  teeth,  and  reading  the 
fraction  on  the  milled  head,  it  can  at  once  be  ascertained  what 
magnifying  power  is  used.  This  is  considered  the  most 
accurate  and  precise  method  of  working,  but  it  is  an  expensive 
piece  of  apparatus,  and  with  care  one  of  the  previous  methods 
named  will  be,  as  a  rule,  sufficient. 

Persons  having  abnormal  vision  are  likely  to  make  errors  in 
measuring.     To  obviate  this,  a  cap  carrying  a  lens   that  will 


126 


MODERN  MICROSCOPY 


correct  the  abnormality  should  be  placed  over  the  top  of  the  eye- 
piece, as  described  on  p.  84,  while  measurements  are  being  taken, 
but  usually  in  eyepieces  arranged  to  carry  micrometers,  and  in 
theRamsden  screw  pattern,  an  adjustment  is  provided  for  sliding 
the  eye-lens  to  and  from  the  micrometer  scale,  so  that  it  may 
be  sharply  focussed. 


Fig.  58. — Botterill's  Trough. 


Troughs,  Live-Cages,  Stage  Forceps,  etc. 

Troughs, — These  are  made  of  various  materials,  including 
glass,  vulcanite,  brass,  etc.,  and  are  used  in  the  examination  of 
infusoria  and  animalcule  alive  under  the  microscope.  The 
essentials  of  a  trough  are  that  a  medium  power,  say  \  inch  at 

least,  can  be  used,  that  it 
may  be  easily  cleaned,  and 
that  if  broken  it  can  be 
repaired.  The  ordinary 
commercial  glass  troughs 
unfortunately  do  not  meet 
these  requirements.  They 
are  difficult  to  clean,  they 
are  invariably  hard  to  mend  when  broken,  and  they  very  often 
leak  when  water  is  put  in.  The  one  that  we  have  found  most 
serviceable  is  the  Botterill's  trough,  as  shown  in  Fig.  58,  which 
consists  of  two  vulcanite  plates  between  which  are  placed  slips  of 
glass,  which  are  separated  by  an  india-rubber  band,  small  bolts 
and  screws  passing  through  the  whole  to  hold  them  together. 
This  is  not  an  ideal  trough,  but  it  certainly  answers  its  purpose 
as  well  as  any  at  present  made. 

Live-Cages. — These  are  not  used  so  largely  for  water  objects  as 
for  insects,  etc.  They  consist  of  a  brass  plate  having  a  glass 
base-plate,  over  which  a  cap  slides,  having  a  very  thin  cover-glass. 
The  subject  to  be  viewed  is  placed  between  these  two  glasses 
and  held  firmly  by  compression.  The  best  form  is  that  designed 
by  Mr.  Rousselet,  shown  in  Fig.  59,  with  which  a  condenser  may 
be  used  conveniently.  It  is  also  so  arranged  that  even  if  a 
specimen  be  fixed  at  the  extreme  edge  of  the  glass  plate,  there  is 
room  for  an  objective  to  work  on  it.  The  ordinary  live-cages 
are  usually  provided  with  a  cover-glass  too  small  in  diameter  for 


ACCESSORY  APPARATUS 


127 


this  to  be  done.  A  very  good  plan  is  often  adopted  by  amateurs 
for  viewing  live  objects  as  follows:  A  square,  flat  piece  of  glass  is 
obtained,  and  on  this  an  india-rubber  ring  is  laid,  into  which  the 
animalcule  can  be  placed ;  a  thin  piece  of  glass  is  now  put  over 
the  top  of  the  india-rubber  ring,  and  this  really  makes  a  very 
serviceable  trough. 

Further  information  on  the  use  of  live-cages,  compressors,  etc., 
will  be  found  on  p.  253. 

Rousselet's  Compressor. — Mr.  Rousselet,  the  designer  of  the  live- 


Fig.  59. — Rousselet's  Live-Cage. 

cage  previously  mentioned,  is  also  the  originator  of  the  most 
efficient  compressor  at  present  obtainable.  It  is  shown  in 
Fig.  60.  The  upper  arm  has  cemented  to  its  under  side  a 
portion  of  a  circle  of  thin  cover-glass,  which  enables  high-power 
objectives  to  be  employed,  and  the  disc  of  glass  in  the  base-plate 
is  not  too  thick  to  prevent  the  employment  of  condensers  of  large 
aperture.  The  compression  is  quite  parallel  in  action,  being 
effected  by  turning  a  milled  head  at  the  top  of  a  drum  containing 
a  spring,  which  causes  the  upper  plate  to  rise  when  the  milled 
head  is  released.  The  cut-off  top  of  the  cover-glass  permits  of 
different   media   being   inserted    while   the  specimen    is    under 


128 


MODEEN  MICROSCOPY 


Fig.  60. — Rousselet's  Com- 

PRESSORltTM. 


examination,  and  the  arm  can  be  turned  aside  when  desired  for 

cleaning,  etc. 

Forceps, — Stage  forceps  are  used  to  hold  unmounted  specimens 

in  the  field  of  view  while  they  are  examined,  there  being  a  fitting 

on  the  forceps  to  go  into  a  hole  pro- 
vided in  the  limb  or  the  stage  of  the 
instrument. 

There  are  in  existence  many  modi- 
fications of  the  apparatus  described 
in  the  foregoing  pages,  the  adoption 
or  rejection  of   which   must   be  left 

to  the  suggestions  which  will  be  naturally  derived  from  practical 

experience.     But  the  forms  of  apparatus  most  commonly  worked 

with,  and  those  whose  merits  particularly  commend  them  to  the 

writer's  judgment,  have  been  described. 

Eye-Shade  for  Monocular  Microscope. 

It  is  recommended  when  working  with  a  monocular  microscope 
that  the  eye  not  actually  employed  should  remain  open.  Many 
workers  experience  no  diffi- 
culty in  doing  this,  but 
others  are  quite  unable  to 
succeed.  For  such  the  eye- 
shade  shown  in  the  accom- 
panying figure  (Fig.  61) 
will  be  found  very  advan- 
tageous. 

It  is  made  of  vulcanite, 
and  consists  of  two  pieces 
jointed  in  the  middle ;  one 
side   is   bored   out   to   the 

outer  diameter  of  the  draw-tube,  over  which  it  slides,  and  the  other 
portion  is  just  a  plain  piece  of  sheet  vulcanite  which  obscures 
light  from  the  disengaged  eye.     It  was  introduced  by  C.  Baker. 

HINTS  REGARDING  THE  CARE  AND  USE  OF 

THE  MICROSCOPE. 

Cleanliness  is  a  most  important  feature  in  microscopical  work. 
Never   allow  dust    to   accumulate  upon   the  microscope,  for    it 


Fig.  61. 


ACCESSOEY  APPARATUS  129 

soon  finds  its  way  between  the  fittings,  and  causes  mechanical 
screws  to  work  with  backlash.  When  not  in  use,  the  instrument 
should  always  be  placed  in  its  case  or  under  a  glass  shade. 

Dusting. — For  dusting  the  microscope,  a  camel's-hair  brush 
should  be  used  in  the  first  place  ;  by  aid  of  this,  dust  can  be 
removed  from  niches  and  crevices  with  great  ease.  For  wiping 
over  the  stand,  also  for  cleaning  eyepiece  lenses,  the  fronts  of 
objectives,  and  other  optical  work,  the  writer  has  always  employed 
handkerchiefs  that  are  made  of  a  mixture  of  silk  and  cotton. 
These  should  be  washed  out  two  or  three  times  until  they  are 
soft  and  free  from  dust. 

Adjusting  a  Microscope. — It  is  not  to  be  recommended  that 
other  than  microscope  makers  should  take  the  instruments  to 
pieces ;  but  it  is  necessary,  where  a  person  resides  abroad,  that 
he  should  be  able  to  adjust  his  own  microscope.     It  is  difficult 
to  give  definite  advice,  because  the  fittings  vary  considerably 
in  every  make  of  microscope.     If  the  rackwork  of  the  coarse 
adjustment  or  sub-stage  develop  loss  of  time,  it  is  more  often 
than  not  due  to  the  bearings  clutching  on  account  of  the  presence 
of  dust,  or  to  their  becoming  dry.     The  way  to  adjust  them  is  as 
follows  :  Rack  the  body  up  as  far  as  it  will  go,  and  mark  lines 
with  a  pen  and  ink   on  the  pinion  stem  and  the  body  of  the 
microscope  to  correspond  with  one  another.     The  object  of  this 
is  to  insure  the  replacement  of  the  body  so  that  the  rack  engages 
the  correct  leaf  of  the  pinion,  and  it  is  here  presumed  that  a 
Jackson  model  microscope  is  used,  and  that  it  has  a  stop-pin  to 
the   rack,  which   prevents   the   body  being   removed   from   its 
fitting.     Now   remove   the    cockpiece,  which   holds  the  pinion 
in  position,  and  take  away  the  pinion  itself,  holding  the  body 
meantime,  or  it  will  run  down  on  to  the  stage. 

Remove  the  body  from  its  fitting,  wipe  both  bearings  and  the 
rack  thoroughly  with  paraffin  oil  and  a  clean  rag,  then  dry  them 
with  another  cloth.  Now  drop  on,  at  most,  two  drops  of  watch- 
makers' oil  on  each  side  of  the  bearing  fittings  attached  to  the 
body,  and  replace  the  tube  in  its  fittings.  It  should  then  be 
moved  up  and  down  until  the  motion  is  quite  free,  and  if  there 
are  adjusting  screws,  they  should  be  so  set  that  there  shall  not 
be  any  shake  in  the  fitting  of  the  body,  but  that  it  may  just,  and 
only  just,  move  in  the  bearings  with  its  own  weight  when  the 

9 


130  MODERN  MICROSCOPE 

instrument  is  set  vertically.  Carefully  wipe  the  pinion  leaves 
out,  and  then,  after  setting  the  ink-marks  in  correspondence 
again,  the  pinion  may  be  attached.  This  usually  has  adjusting 
screws  to  the  cockpiece,  which  push  it  closer  to,  or  allow  it  to 
remain  farther  from,  the  rack  ;  these  should  be  so  set  as  to  give 
a  soft  movement.  It  is  useless  to  attempt  this  procedure  with  a 
microscope  that  never  has  worked  well,  but  where  an  instrument 
has,  after  use,  become  unsatisfactory  in  the  mechanical  parts,  it 
generally  is  found  to  answer.  Practically  the  same  treatment 
is  applicable  to  the  mechanical  movements  of  the  stage,  but  very 
great  care  is  essential,  lest  either  of  the  plates  become  bent,  an 
accident  that  is  more  easy  of  occurrence  than  would  be  deemed 
likely. 

Objectives. — It  is  unwise  for  unskilled  persons  to  unscrew 
parts  of  microscopic  objectives  ;  they  are  frequently  deranged  by 
this  means.  If  at  any  time  it  should,  from  some  cause  or  another, 
be  necessary  to  unscrew  them,  an  ink-mark  or  small  scratch 
should  be  made  on  each  combination,  so  that  when  put  together 
again  they  can  be  screwed  up  in  the  same  position  as  before. 

After  using  an  oil  immersion  objective,  the  oil  must  be  carefully 
removed  from  the  front  lens  by  wiping  with  the  handkerchief. 
Undue  pressure  must  not  be  used,  but  it  must  be  thoroughly 
cleaned.  If  oil  should  become  dried  on  the  front  lens  at  any 
time,  it  will  be  best  to  place  some  fresh  immersion  oil  over  it ; 
it  should  then  be  allowed  to  stand  in  a  place  free  from  dust  for 
about  an  hour,  when  the  whole  may  be  cleaned  off  together. 

Never  use  the  immersion  oil  supplied  for  objectives  for  lubri- 
cating the  microscope  fittings.  It  is  resinous,  and  will  completely 
spoil  the  movements. 

Do  not  touch  the  microscope  with  hands  soiled  by  reagents, 
and  care  should  be  taken  not  to  spill  any  media  on  the  instru- 
ment, and  particularly  the  stage. 

If  dust  specks  are  seen  in  the  field,  the  eyepiece  should  be 
rotated,  and  if  the  specks  are  in  it  they  also  will  revolve.  If 
they  remain  stationary  they  may  either  be  in  the  objective  or  in 
the  lamp-glass  ;  their  location  can  then  be  ascertained  by  moving 
the  mirror,  when,  if  the  specks  move  too,  they  are  not  in  the 
objective.  Dust  can  be  removed  from  the  eyepiece  lenses  by 
wiping  with  a  handkerchief  dipped  in  spirits  of  wine. 


ACCESSOEY  APPARATUS  131 

Work  with  the  eye  close  to  the  eye-lens  of  the  eyepiece  to  get 
the  best  results. 

Never  use  high-power  eyepieces  when  low  ones  will  do. 

Be  careful  not  to  knock  objectives,  eyepieces,  or  condensers,  or 
let  them  fall. 

Travelling. — When  travelling  with  a  microscope  it  is  always 
well  to  pack  the  instrument  round  with  tissue-paper,  so  that  it 
cannot  shake  in  its  case.  Screws  frequently  become  loosened,  and 
in  some  instances  broken,  and  movements  disordered,  by  severe 
shaking  while  in  transit. 

For  a  more  exhaustive  treatment  of  the  subject  the  following 
textbooks  are  recommended : 

Carpenter's  '  The  Microscope  and  its  Revelations,'  edited  by 
the  Rev.  W.  H.  Dallinger,  LL.D.,  F.R.S.,  etc. 

'Microscopy,'  by  Dr.  E.  J.  Spitta. 

'Photo-micrography,'  by  Dr.  E.  J.  Spitta. 

'Photo-micrography,'  by  Mr.  Andrew  Pringle. 

'Practical  Photo-Microscopy,'  by  J.  Edwin  Barnard, F.R.M.S. 

And  '  Practical  Microscopy,'  a  handbook  for  beginners,  by 
Dr.  F.  Shillington  Scales. 

As  periodicals  the  best  are  :  Knowledge,  published  at  42,  Blooms- 
bury  Square,  London,  W.C.,  which  devotes  special  columns  to 
the  subject ;  the  Journal  of  the  Royal  Microscopical  Society , 
published  bi-monthly,  and  the  Journal  of  the  Quekett  Club,  pub- 
\ished  twice  a  year,  both  at  20,  Hanover  Square,  London,  W. 

The  Choice  of  an  Outfit. 

Summarizing  the  conclusions  arrived  at  in  the  foregoing  pages, 
it  may  be  well  to  briefly  recapitulate  some  points  to  be  borne  in 
mind  in  selecting  an  outfit. 

The  Microscope-Stand. — In  the  interests  of  the  advancement 
of  microscopy  as  a  science,  the  best  and  most  suitable  means 
must  be  commended,  and  nothing  can  be  gained  by  encouraging 
the  perpetuation  of  such  instruments  as  do  not  embody  the 
accuracy  of  adjustment  or  convenience  of  design  which  the 
modern  worker  with  his  beautifully  perfect  optical  accessories 
actually  needs  in  order  to  derive  all  the  benefit  that  his  lenses 
are  capable  of  yielding. 

It  is  asserted  by  users  of  Continental  microscopes,  whose  name 


132  MODERN  MICROSCOPY 

is  legion,  that  the  British  microscope  exceeds  the  needs  of  the 
laboratory  worker.  The  only  response  from  the  expert  to  such  a 
criticism  is  that  there  is  a  want  of  appreciation  and  education 
in  matters  microscopical  in  the  laboratory.  It  is  impossible  to 
disregard  the  modern  spirit  which  demands  excessive  rapidity  of 
work  at  the  cost  of  excellence  and  accuracy,  and  it  is  not  too 
much  to  say  that  the  man  who  examines  structures  with  a  good 
TV-inch  oil  immersion  objective  and  an  Abbe  illuminator  has 
limited  his  knowledge  to  an  extent  which  would  cause  him  great 
surprise  if  he  had  but  the  opportunity  of  seeing  the  same  subject 
properly  illuminated  with  a  sub-stage  condenser  having  a  suit- 
able ratio  of  aperture  to  that  of  the  objective  and  the  microscope 
properly  manipulated. 

Notwithstanding  the  great  strides  which  have  been  made  in 
recent  years  by  Continental  makers,  particularly  in  the  direction 
of  the  improvement  of  the  fine  adjustment,  the  instruments  of 
English  manufacture  alone  combine  all  those  refinements  which 
have  been  found  so  advantageous  by  critical  workers  and  have 
been  designed  and  provided  with  such  excellent  judgment.  The 
Continental  makers  have  yet  to  produce  instruments  having 
sub-stages  with  centring  screws,  and  excepting  in  two  instances, 
mechanical  stages  have  yet  to  be  built  as  a  part  of  the  instru- 
ment, and  not  as  an  auxiliary  attachment. 

Our  preference  would  therefore  be  for  a  microscope  by  one  of 
the  well-known  English  makers,  and  the  following  would  be  the 
order  of  preference  for  the  various  mechanical  fittings : 

1.  Coarse  adjustment  by  rackwork. 

2.  Fine  adjustment. 

3.  Compound  sub-stage,  with  screws  to  centre. 

4.  Mechanical  movements  for  the  stage. 

5.  Mechanical  draw- tube. 

6.  Fine  adjustment  to  sub-stage. 

7.  Concentric  rotation  to  the  stage. 

8.  Divided  scales,  as  may  be  found  necessary. 

9.  Other  mechanical  fittings,  such  as  centring  screws  and 
rackwork  to  the  rotation  of  the  stage,  rackwork  rotation  to 
sub-stage,  etc. 

In  amplification  of  the  above  we  would  remark  that  where 
questions  of   economy  prevail,  the  sub-stage  may  be  replaced 


ACCESSOEY  APPARATUS  133 

with  an  under-fitting  having  centring  screws,  and  the  mechanical 
stage  with  a  sliding-bar. 

Many  microscopes  are  made  in  plain  form  as  a  foundation 
on  which,  as  a  superstructure,  many  of  the  mechanical  fittings 
can  be  subsequently  mounted.  Consideration  might  with  pro- 
priety be  given  by  a  beginner  to  such  instruments. 

On  no  account  purchase  a  microscope  which  has  not  its 
fittings — that  is,  the  eyepiece,  sub-stage,  and  objective  screw  of 
the  Royal  Microscopical  Society's  standard  gauges. 

The  Condenser. — Bearing  in  mind  the  strictures  that  have 
been  passed  on  the  Abbe  illuminator,  it  is  well  on  the  threshold 
of  work  to  choose  a  condenser  that  will  be  of  permanent  utility, 
and  although  the  Abbe  illuminator  is  sufficiently  effective  with  a 
J-inch  objective  of  medium  aperture,  and  with  its  top  lens  removed 
will  work  with  low-power  objectives,  it  is  not  sufficiently  well 
corrected  to  develop  the  possibilities  of  a  y^-inch  oil  immersion 
objective.  It  will  always  be  worth  while,  therefore,  to  begin 
with  a  well-corrected  achromatic  condenser. 

Objectives. — Having  decided  whether  or  no  the  students' 
series  or  the  higher  class  objectives  are  to  be  purchased,  the 
powers  that  should  be  chosen  would  depend  on  the  work  that 
was  to  be  undertaken.  For  general  botanical  and  biological 
work,  1-inch  or  f-inch,  and  J-inch  are  the  most  useful.  In  the 
majority  of  laboratories  the  f-inch  is  preferred.  For  bacterio- 
logical work  a  TV-inch  oil  immersion  is  added  to  the  above. 

The  amateur  does  the  greater  part  of  his  work  with  low 
powers,  and  he  will  be  well  advised  to  choose  at  the  beginning 
a  2-inch,  1-inch,  and  J-inch. 

Eyepieces.— These  should  not  be  chosen  too  high  in  power, 
for  they  will  be  found  of  little  use.  The  greater  part  of  the 
work  is  done  with  a  No.  2  eyepiece.  If  a  second  eyepiece  is  taken, 
it  should  be  either  a  No.  3  or  No.  4,  the  higher  powers  being  only 
required  for  testing  the  quality  and  performance  of  objectives. 

Sundry  Accessories. — All  workers  will  find  it  an  advantage 
to  take  a  revolving  nosepiece  to  carry  two  or  three  objectives, 
and  amateurs  will  find  the  inclusion  of  a  bull's-eye  condenser 
for  the  examination  of  opaque  subjects,  troughs  for  water  speci- 
mens, together  with  a  live -cage  and  compressor,  useful  and 
advantageous  accessories. 


CHAPTER  V 

A  SHOET  NOTE  CONCEENING  THE  INFLUENCE 

OF    DIFFBACTION    ON    THE    EESOLVING    POWEE    OF 

MICEOSCOPICAL  OBJECTIVES,  AND    ON  THE 

APPAEENT  COLOUE  OF  MICEOSCOPICAL  OBJECTS 

By  the  late  Dr.  G.  JOHNSTONE  STONEY,  F.R.S. 

If  we  look  from  a  distance  at  a  flame  through  a  thin  feather  or 
other  uniformly  ruled  grating  we  see  the  flame,  and  around  or  on 
either  side  of  it  a  number  of  lateral  coloured  images,  which  are 
wider  and  usually  fainter  the  farther  out  that  they  lie.  We  thus 
learn  that  the  light  which  passes  through  the  grating  becomes 
both  a  direct  beam  and  a  number  of  lateral,  or  diffracted,  beams, 
as  they  are  called.  The  proportions  in  which  the  light  which 
passes  the  grating  is  distributed  between  the  central  beam  and 
the  several  diffracted  beams  depends  upon  the  ratio  of  the  widths 
of  the  openings  to  the  widths  of  the  bars  of  the  grating,  as  well 
as  upon  such  particulars  as  whether  each  opening  is  a  mere  hole 
and  each  bar  a  mere  obstruction,  or  whether  they  are  occupied 
by  material  which  acts  on  the  light,  especially  if  it  act  like  a 
prism.  It  rarely  happens  that  this  distribution  does  not  per- 
ceptibly differ  for  light  of  different  wave-lengths.  The  direct 
beam  consists  of  light  in  very  nearly  the  same  state  as  if  it  had 
passed  through  a  simple  opening  of  the  size  of  the  grating,  ex- 
cept that  it  is  fainter — usually  fainter  in  some  colours  than  in 
others. 

Accordingly,  if  the  eye  when  looking  at  the  grating,  or  if  the 
object-lens  of  a  telescope,  were  to  receive  only  this  central  beam 
wherewith  to  form  an  image  of  the  grating,  the  image  would  be 

almost  identical  with  that  which  would  be  furnished  by  light 

134 


THE  INFLUENCE  OF  DIFFRACTION  135 

coming  through  an  opening  covered  by  tinted  glass,  and  no  trace 
of  the  ruling  would  be  seen  in  it.  In  order  to  see  the  ruling,  the 
telescope  lens  must  be  able  to  catch  and  forward  to  its  focus 
other  rays  which  have  passed  through  the  grating  than  those  of 
the  central  beam.  The  more  of  the  lateral  beams  which  it  can 
transmit  and  combine  at  its  focus  with  the  light  of  the  central 
beam  (where  they  will  by  interference  strengthen  some  parts  of 
the  image  formed  by  the  central  beam,  and  enfeeble  others,  thus 
introducing  detail),  the  more  nearly  will  the  image  it  forms 
resemble  the  actual  grating  in  detail,  and  in  freedom  from  false 
colour.  If  it  succeeds  in  catching,  along  with  the  central  beam, 
even  some  small  portion  of  the  nearest  of  the  diffraction  beams, 
the  image  will  exhibit  lines,  and  the  proper  number  of  lines, 
though  it  will  not  present  correctly  such  minuter  detail  as  the 
widths  of  the  lines  and  of  the  spaces  between  them. 

Cases  exactly  analogous  to  this  occur  with  the  microscope. 
"When  an  object  covered  with  dots,  such  as  Pleurosigma  angu- 
latum,  has  been  focussed  upon  the  stage,  and  is  resolved,  the 
diffraction  beams  may  be  clearly  seen  upon  the  back  lens  of  the 
objective  by  removing  the  eyepiece  and  looking  down  the  tube. 
With  this  diatom  there  will  then  be  seen  the  central  beam,  and 
portions  of  the  nearest  of  the  lateral  beams,  six  in  number.  A 
rather  small  cone  of  illumination  is  best  to  show  them  con- 
spicuously if  white  light  be  used,  and  they  can  be  seen  with 
larger  cones  of  illumination  and  very  sharply  defined  if  mono- 
chromatic green  light,  produced  by  prisms,  be  employed. 

The  markings  on  the  Pleurosigma  angulatum  are  spaced  in 
each  row  at  intervals  which  have  been  measured,  and  found  to 
be  equal  to  wave-lengths  of  red  light.  With  so  close  a  ruling 
the  lateral  beams  are  much  diffracted  or  bent  aside,  and  dry 
objectives  can  only  take  in  the  central  beam  and  a  portion  of 
each  of  the  nearest  diffracted  beams.  This  enables  us  to  see 
with  such  an  objective  the  markings  correctly  so  far  as  concerns 
their  number  and  positions,  but  any  further  detail  is  not  correctly 
presented.  Immersion  objectives  can  transmit  nearly  the  whole 
of  the  six  nearest  lateral  beams,  which  are  those  that  would 
produce  spectra  of  the  first  order.  We  now  see  some  detail :  the 
dots  appear  hexagonal,  and  are  separated  from  one  another  by 
walls  which  are  thin,  and  which  look  like  a  honeycomb.    This  is 


136  MODERN  MICROSCOPY 

the  first  and  the  only  step  we  can  take  towards  learning  what  the 
actual  detail  upon  this  diatom  is,  since  no  objective  is  competent 
to  supply  to  the  image  the  second  or  subsequent  diffraction 
beams  ;  inasmuch  as  no  immersion  fluid  can  shorten  the  waves 
of  visible  light  so  much  as  would  enable  the  object  to  emit  and 
the  lens  to  receive  these  further  diffraction  beams. 

The  unequal  distribution  of  colour  between  the  several  beams 
is  strikingly  exhibited  by  the  diatom  known  as  Actinocylus  Half  sir. 
The  phenomenon  may  be  conveniently  examined  through  a  i-inch 
apochromatic,  over  which  is  mounted  an  iris  diaphragm — an 
adjunct  which  is  useful  for  many  purposes.  Select  a  frustule 
which  is  blue  when  this  upper  diaphragm  is  partly  closed. 
Remove  the  eyepiece,  close  the  lower,  and  open  the  upper  dia- 
phragm. Then,  on  looking  down  the  tube,  it  will  be  seen  that 
most  of  the  red  is  located  in  the  ring  of  first  lateral  beams,  with, 
of  course,  an  equal  defect  of  red  in  the  central  beam.  Hence 
the  blue  colour  seen  when  the  image  is  formed  by  the  central 
beam  only.  Now  place  a  small  central  stop  (which  may  be  cut 
out  of  card)  over  the  back  lens  of  the  objective,  open  the  upper, 
and  partially  close  the  lower  diaphragm.  The  image  is  then 
formed  by  the  ring  of  lateral  beams  only,  and  will  be  found  to  be 
preponderatingly  red. 


PART   IT 


CHAPTER  VI 

INTBODUCTION 

In  publishing  methods  of  preparing,  staining,  hardening, 
and  mounting  microscopic  objects,  I  have  adopted  the 
system  employed  in  my  classes  for  some  years  past — that 
is,  each  separate  stage  of  procedure  is  arranged  in  succes- 
sive lessons  or  chapters.  A  subject  such  as  this  cannot 
be  so  lucidly  described  in  writing  as  by  demonstration, 
but  it  has  been  my  aim  to  make  it  as  clear  as  possible, 
so  that  if  the  instructions  are  carefully  followed  and 
practised,  successful  permanent  work  can  be  performed  ; 
but  it  is  only  by  most  scrupulous  care  and  constant 
practice  that  any  degree  of  success  in  this  work  can  be 
attained. 

In  books  on  this  and  cognate  subjects  it  too  often 
happens  that  tools,  instruments,  and  routine  are  pre- 
scribed that  tend  to  make  work  needlessly  laborious  and 
expensive,  and  are  in  consequence  causes  of  discourage- 
ment to  the  readers.  The  directions  given  in  the  succeed- 
ing pages  will,  it  is  believed,  commend  themselves  for  their 
directness  and  simplicity.  They  are,  moreover,  thoroughly 
practical,  and  are  the  processes  that  I  have  found  the 
most  effective  after  more  than  thirty  years'  experience  as 

a  mounter  of  microscopic  objects. 

137 


138 


MODERN  MICEOSCOPY 


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CHAPTER  VII 

HAEDENING  AND  PEESEEVING  ANIMAL  TISSUES  AND 
OEGANS  FOE  MICEOSCOPICAL  EXAMINATIONS 

Fresh  untreated  tissues  are  unsuited  for  microscopical  purposes, 
but  it  is  sometimes  advisable  to  observe  the  appearance  of  some 
specimens,  such  as  muscle-fibres,  tendon,  connective  tissues,  and 
nerve-fibres,  while  fresh.  When  this  is  desired,  the  tissue  must 
be  examined  in  certain  fluids  called  'normal  fluids,'  that  will  alter 
its  character  as  little  as  possible.  Those  generally  used  are  :  (1) 
Blood  serum  ;  (2)  the  aqueous  humour  from  a  fresh  eye  ;  and  (3) 
normal  or  j  per  cent,  salt  solution.  The  two  former  are  difficult 
to  obtain,  but  the  latter  can  be  made  at  any  time,  and  it  will 
answer  for  most  purposes.  Place  a  small  piece  of  the  tissue  on 
a  slide,  add  a  drop  or  two  of  salt  solution,  take  two  needles  fixed 
in  holders  and  carefully  separate  the  fibres  from  each  other ; 
this  process  is  called  teasing.  When  sufficiently  teased,  apply 
a  cover-glass  and  examine.  You  may  now  wish  to  irrigate  with 
some  staining  reagent ;  if  so,  place  a  few  drops  of  the  stain  at 
one  edge  of  the  cover-glass,  and  apply  a  piece  of  blotting-paper 
to  the  other  side ;  this  will  absorb  the  salt  solution,  and  the 
staining  fluid  will  follow  and  take  its  place  around  the  tissue ; 
the  slide  may  then  be  placed  under  the  microscope,  and  the 
action  of  the  reagent  observed. 

These  specimens  cannot,  as  a  rule,  be  kept.  For  permanent 
preparations  the  tissues  or  organs  must  be  hardened.  This 
is  accomplished  by  subjecting  them  to  the  action  of  certain 
hardening  or  fixing  solutions.  The  following  are  most  com- 
monly used : 

Absolute  Alcohol.  —  Suitable  for  stomach,  pancreas,  and 
salivary  glands.     These  organs   must   be   perfectly  fresh,  and 

142 


HAPPENING  AND  PKESEKVING  ANIMAL  TISSUES     143 

they  should  be  cut  into  small  pieces,  so  that  the  alcohol  may 
penetrate  as  quickly  as  possible. 

Change  the  alcohol  every  day  for  the  first  three  days.  The 
hardening  is  usually  complete  in  a  week. 

Chromic  Acid  and  Spirit. — Chromic  acid  J  per  cent.,  watery 
solution  2  parts,  and  methylated  spirit  1  part.  This  reagent 
hardens  in  about  ten  days.  Then  transfer  to  methylated  spirit, 
which  should  be  changed  every  day  until  no  colour  comes  away 
from  the  tissues.  It  is  suitable  for  cartilage,  nerve-trunks,  heart, 
lips,  bloodvessels,  trachea,  lung,  tongue,  bladder,  ureter,  intes- 
tines, and  oesophagus. 

Potassium  Bichromate. — Make  a  2  per  cent,  watery  solution. 
This  will  harden  in  about  three  weeks.  Then  transfer  to  methy- 
lated spirit,  and  change  the  spirit  every  day  until  no  colour 
comes  away  from  the  tissues.  It  is  suitable  for  muscle,  spleen, 
liver,  and  kidney. 

Ammonium  Bichromate. — Make  a  2  per  cent,  watery  solution. 
It  hardens  in  from  three  to  four  weeks.  Then  transfer  to 
methylated  spirit,  and  change  every  day  until  no  colour  comes 
away  from  the  tissues.  It  is  suitable  for  spinal  cords,  medulla, 
pons  Varolii,  cerebellum,  and  cerebrum. 

Miiller's  Fluid. — Bichromate  of  potash  30  grains,  sulphate  of 
soda  15  grains,  distilled  water  3J  ounces.  It  hardens  in  from 
three  to  five  weeks.  Then  transfer  to  methylated  spirit,  and 
change  every  day  until  no  colour  comes  away  from  the  tissues. 
Suitable  for  lymphatic  glands,  eyeballs,  retina,  and  thymus 
gland. 

Methylated  Spirit. — May  be  used  universally,  if  preferred, 
but  it  has  a  tendency  to  shrink  some  tissues  too  much.  It 
hardens  in  about  ten  days.  Change  the  spirit  every  twenty- 
four  hours  for  the  first  three  days.  Suitable  for  skin,  scalp, 
testicle,  penis,  prostate  gland,  vas  deferens,  epididymis,  ovary, 
uterus,  Fallopian  tubes,  placenta,  mammary  gland,  supra-renal 
glands,  tonsils,  and  all  injected  organs. 

Decalcifying  Solution. — For  bones.  Make  a  J-  per  cent, 
watery  solution  of  chromic  acid,  and  for  every  ounce  add  5 
drops  of  nitric  acid.  This  fluid  will  soften  the  femur  of  a  dog 
in  about  three  weeks ;  larger  bones  will  take  longer.  Change 
the  fluid  several  times,  and  test  its  action  by  running  a  needle 


144  MODEKN  MICROSCOPY 

through  the  thickest  part  of  the  bone.  If  it  goes  through  easily, 
the  bone  is  soft  enough ;  if  not,  continue  the  softening  process 
a  little  longer.  When  soft  enough,  transfer  to  water,  and  soak 
for  an  hour  or  two ;  then  pour  off  the  water  and  add  a  10  per 
cent,  solution  of  bicarbonate  of  soda,  and  soak  for  twelve  hours 
to  remove  all  trace  of  acid.  Wash  again  in  water,  and  place  in 
methylated  spirit  until  required.  Bones  and  teeth  should  always 
be  softened  in  a  large  quantity  of  the  decalcifying  solution. 

Olfactory  Region. — Divide  with  a  saw  the  head  of  a  freshly 
killed  rabbit  or  guinea-pig  longitudinally,  and  parallel  with  the 
nasal  septum.  Cut  out  the  septum  so  as  to  expose  the  olfactory 
region,  which  is  recognized  by  its  brown  colour.  Dissect  out  a 
portion  including  some  of  the  turbinated  bones.  Harden  this 
in  Midler's  fluid  for  three  or  four  days.  Then  transfer  to 
chromic  and  nitric  acid  decalcifying  solution,  and  soak  until 
the  bones  are  quite  soft.  W^ash  well  in  water  to  remove  all 
trace  of  acid,  and  complete  the  hardening  in  methylated  spirit. 

Cochlea. — Dissect  out  the  internal  ear  of  a  freshly  killed 
young  guinea-pig,  open  bulla  with  bone-forceps,  when  a  conical 
elevation,  the  cochlea,  will  be  seen.  Eemove  as  much  of  the 
surrounding  bone  as  possible,  and  place  the  cochlea  in  Miiller's 
fluid  for  two  weeks  to  harden  the  delicate  nervous  tissues.  Then 
transfer  to  chromic  and  nitric  acid  decalcifying  solution,  and 
soak  until  the  bone  is  soft.  Place  in  weak  spirit  for  a  day  or 
two,  and  then  transfer  to  strong  methylated  spirit. 

Corrosive  Sublimate. — Tissues  may  be  fixed  very  quickly  in 
corrosive  sublimate.  Make  a  saturated  solution  in  5  per  cent, 
glacial  acetic  acid.  The  specimens  should  be  removed  from  the 
solution  as  soon  as  they  are  fixed,  directly  they  become  opaque 
throughout.  Then  wash  in  repeated  changes  of  70  per  cent, 
alcohol,  to  which  a  little  tincture  of  iodine  has  been  added.  This 
process  will  fix  tissues  in  a  few  minutes. 

Picric  Acid. — Make  a  saturated  solution  in  water.  This 
solution  will  fix  small  pieces  of  tissue  in  a  few  minutes ;  larger 
specimens  will  require  from  three  to  six  hours'  immersion. 
Then  wash  out  the  picric  acid  with  repeated  changes  of  spirit. 
Water  must  not  be  used,  as  it  is  hurtful  to  the  tissues  that 
have  been  prepared  by  this  method.  For  the  same  reasons, 
during   all   subsequent   stages   of   treatment,  water   should   be 


HARDENING  AND  PRESERVING  ANIMAL  TISSUES     145 

avoided,  and  the  staining  should  be  carried  out  in  alcoholic 
solutions. 

Formaldehyde. — This  may  be  used  universally  if  desired. 
It  is  sold  commercially  as  '  formal '  in  a  40  per  cent,  solution. 
This  must  be  reduced  by  the  addition  of  water  to  a  2  or  4  per 
cent,  solution.  It  is  specially  useful  for  hardening  nervous  tissues 
and  for  eyes ;  the  latter  are  completely  hardened  in  twenty-four 
hours. 

When  in  great  haste,  tissues  may  instantly  be  fixed  in  boiling 
water.  Boil  some  water  in  a  test-tube,  then  drop  in  small  pieces 
of  the  tissue,  and  boil  again  for  a  few  seconds.  The  specimen 
may  then  be  placed  at  once  in  gum  and  syrup,  and  when  pene- 
trated, freeze,  and  make  the  sections.  This  method  should  only 
be  used  when  a  section  is  urgently  wanted. 


General  Directions  for  Hardening  Tissues. 

1.  Always  use  fresh  tissues. 

2.  Cut  the  organs  into  small  pieces  with  a  sharp  knife. 

3.  Never  wash  a  specimen  in  water ;  when  it  is  necessary  to 

remove  any  matter,  allow  some  normal  salt  solution  to 
flow  over  the  surface  of  the  tissue,  or  wash  in  some  of  the 
hardening  reagent  you  are  going  to  use. 

4.  All  specimens  should  be  hardened  in  a  large  quantity  of 

the  reagent ;  too  many  pieces  should  not  be  put  into  the 
bottle,  and  they  should  be  kept  in  a  cool  place. 

5.  In  all  cases  the  hardening  process  must  be  completed  in 

spirit. 

6.  Label  the  bottles,  stating  the  contents,  the  hardening  fluid 

used,  and  when  changed.  Strict  attention  to  these  details 
is  necessary  for  successful  histological  preparations,  for  if 
the  hardening  is  neglected  good  sections  cannot  be  made. 


10 


CHAPTER  VIII 

EMBEDDING  TISSUES  AND  SECTION-CUTTING 

To  Cut  Sections  with  a  Razor  by  Hand. — Take  the  tissue 
between  the  thumb  and  forefinger  of  the  left  hand.  Hold  the 
finger  horizontally,  so  that  its  upper  surface  may  form  a  rest  for 
the  razor  to  slide  on.  Take  the  razor,  hold  it  firmly  in  the  hand, 
keep  the  handle  in  a  line  with  the  blade,  and  draw  it  through 
the  tissue  from  heel  to  tip  towards  yourself.  While  cutting,  keep 
the  razor  well  wetted  with  dilute  methylated  spirit,  and  as  the 
sections  are  cut  place  them  in  a  saucer  of  dilute  methylated 
spirit. 

Embedding  in  Paraffin  Wax  and  Lard. — Melt  together  by 
the  aid  of  gentle  heat  four  parts  of  solid  paraffin  and  one  part 
of  lard.  A  quantity  of  this  may  be  made  and  kept  ready  for  use 
at  any  time.  Melt  the  paraffin  mass  over  a  water-bath.  Take 
the  specimen  and  dry  it  between  the  folds  of  a  cloth  to  remove 
the  spirit,  so  that  the  paraffin  may  adhere  to  its  surfaces,  place  it 
in  a  pill-box  in  the  desired  position,  and  pour  in  enough  melted 
paraffin  to  cover  it,  then  set  aside  to  cool.  When  quite  cold, 
break  away  the  pill-box  and  cut  sections  from  the  embedded  mass 
with  a  sharp  razor.  When  a  number  of  specimens  are  embedded, 
and  it  is  desired  to  keep  them  for  some  time,  they  should  be 
preserved  in  a  jar  of  methylated  spirit. 

To  Infiltrate  a  Tissue  with  Paraffin. — Dehydrate  the  speci- 
men in  absolute  alcohol  for  several  hours,  then  transfer  to  chloro- 
form or  xylol,  in  which  it  must  remain  until  perfectly  saturated. 
When  clear,  place  in  a  bath  of  melted  paraffin  of  45°  C.  (melting- 
point),  and  keep  it  at  this  temperature  for  several  hours,  so  that 
the  paraffin  may  penetrate  to  the  middle  of  the  tissue.  Then 
remove  it  from  the  paraffin  and  put  it  into  a  small  pill-box,  pour 

146 


EMBEDDING  TISSUES  AND  SECTION-CUTTING     147 


in  enough  paraffin  to  fill  the  box,  and  as  the  paraffin  cools,  add  a 
little  more  to  make  up  the  shrinkage  and  set  aside  to  cool.  When 
cold,  place  in  water  for  a  few  minutes  ;  this  will  soften  the  paper, 
and  facilitate  the  removal  of  the  pill-box.  You  will  now  have  a 
cylinder  of  paraffin  with  the  specimen  firmly  fixed  in  its  centre, 
and,  if  desired,  the  paraffin  may  be  pared  away  from  the  sides 
until  a  square  block  is  obtained.  The  sections  may  now  be  made 
by  hand  with  a  razor,  or  the  block  can  be  fixed  to  a  microtome 
with  a  little  melted  paraffin. 
The  sections  must  be  placed  in 
turpentine  to  remove  the  paraffin, 
then  in  absolute  alcohol  to  re- 
move the  turpentine,  and  finally, 
in  distilled  water  to  remove  the 
alcohol  ;  they  may  then  be 
stained.  Sometimes  it  is  de- 
sirable to  stain  the  tissue  in 
bulk  before  it  is  embedded.  In 
this  case  the  sections  need  only 
go  into  turpentine  or  benzole  to 
wash  away  the  paraffin ;  they 
may  then  be  mounted  in  Canada 
balsam. 

The  above  process  requires  an 

.     iv        t     ,,       mi  •    •  11       Fig.  62. — Potato-Steamer  converted 

embedding  bath.    This  is  usually  INT0  AN  embedding  Bath. 

an  expensive  affair,  but  One  that    A>  Thermometer ;  B,  test-tubes  ;  C,  disc 

will  answer  all  ordinary  purposes      °f  tin  ;  D>  tin  supports ;  E,  water ; 

r,    cotton-wool  ;    G,    spirit   or   small 

can  easily  be  made.  paraffin  lamp. 

Get  a  small  potato-steamer, 
and  cut  a  hole  in  the  lower  vessel  to  admit  a  spirit  or  small 
paraffin  lamp.  Get  a  tinsmith  to  cut  out  a  circular  plate  of  tin 
to  fit  into  the  upper  vessel,  in  which  some  holes  must  be  cut 
to  take  the  test-tubes,  and  to  the  sides  of  the  vessel  four  small 
pieces  of  tin,  bent  at  right  angles,  must  be  soldered  to  support 
the  tin  plate.  A  piece  of  tin  must  also  be  soldered  over  the 
perforated  bottom  of  the  vessel,  so  that  it  will  hold  water.  When 
the  alterations  are  complete,  place  a  layer  of  cotton-wool  or  a 
piece  of  felt  on  the  bottom  of  the  steamer,  to  protect  the  test- 
tubes  from  breakage;  half  fill  with  water,  add  a  thermometer, 


148 


MODERN  MICROSCOPY 


light  the  lamp,  and  on  the  desired  temperature  being  attained, 
put  some  paraffin  in  the  test-tubes,  place  them  in  the  steamer, 
and  when  the  paraffin  has  melted  add  the  specimens. 

After  use  dry  the  apparatus  so  that  rust  may  not  set  in.  If 
this  is  attended  to  it  will  last  for  years. 

When  a  proper  embedding  bath  cannot  be  obtained,  tissues 
may  be  infiltrated  with  paraffin  in  the  following  way  :  Dehydrate 
the  specimen  in  absolute  alcohol  ;  then  place  in  a  quantity  of 
chloroform  or  benzole,  ten  or  twelve  times  the  bulk  of  the  tissue, 
until  saturated  ;  add  small  pieces  of  paraffin  until  no  more  will 
dissolve,  and  set  aside  for  several  ^ours.  Apply  gentle  heat  to 
drive  off  the  solvent  and  melt  the  paraffin,  after  which  the  tissue 


Fig.  63. — Cole's  Pattern  Microtome. 


can  be  removed  and  embedded  in  a  pill-box  of  paraffin  of  the 
desired  melting-point. 

Cole's  Microtome  and  Embedding  in  Carrot. — When  a 
number  of  sections  are  wanted,  or  when  a  complete  section  of 
an  organ  is  desired,  a  microtome  should  be  used.  A  very  good 
and  simple  instrument  can  be  obtained  from  Messrs.  Watson 
and  Sons,  313,  High  Holborn.  Screw  the  microtome  firmly  to 
the  table,  and  with  the  brass  tube  supplied  with  the  microtome 
punch  out  a  cylinder  of  carrot  to  fit  into  the  well  of  the  micro- 
tome. Cut  this  in  half  longitudinally,  and  scoop  out  enough 
space  in  one  half  of  the  carrot  to  take  the  specimen  ;  then  place 
the  other  half  of  carrot  in  position,  and  make  sure  that  the 
specimen  is  held  firmly  between  them,  but  it  must  not  be 
crushed.     Now  put  the  cylinder  of  carrot  and  specimen  into  the 


EMBEDDING  TISSUES  AND  SECTION-CUTTING     149 

well  of  the  microtome' and  commence  cutting  the  sections.  A 
good  razor  will  do,  but  it  is  better  to  use  the  knife  which  Messrs. 
Watson  supply  with  the  microtome.  While  cutting,  keep  the 
knife  and  plate  of  the  microtome  well  wetted  with  dilute  methy- 
lated spirit,  and  as  the  sections  are  cut  place  them  in  a  saucer 
of  dilute  spirit.  A  number  of  sections  may  be  cut  and  preserved 
in  methylated  spirit  until  required. 

When  a  specimen  has  a  very  irregular  outline,  it  cannot  be 
successfully  embedded  in  carrot.  Paraffin  should  then  be  used. 
Place  the  tissue  in  the  well  of  the  microtome  in  the  desired 
position,  pour  in  enough  melted  paraffin  to  cover  it,  and  when 
cold  cut  the  sections. 

Freezing  Microtome.— Cathcart's  is  the  most  simple  and 
cheapest  freezing  microtome,  and  it  can  be  obtained  from  any 
optician. 

1.  Cut  a  slice  of  the  specimen  about  J  inch  thick,  in  the 
direction  you  wish  to  make  the  section. 

2.  Place  in  water  for  an  hour  to  remove  the  alcohol. 

3.  Transfer  to  a  mixture  of  gum-water  5  parts,  saturated 
watery  solution  of  loaf-sugar  3  parts,  and  allow  it  to  soak  in 
this  for  about  twelve  hours ;  or,  if  a  few  drops  of  carbolic  acid 
are  added  to  the  mixture,  tissues  may  remain  in  it  for  months 
without  harm. 

4.  Clamp  the  microtome  to  a  table,  fix  the  ether  spray  in  its 
place,  and  fill  the  bottle  with  ether.  Methylated  ether,  specific 
gravity  720,  will  do. 

5.  Put  a  little  gum  and  syrup  on  the  zinc  plate  of  the 
microtome,  and  place  the  tissue  in  it.  Commence  working  the 
bellows,  and  as  soon  as  all  the  gum  has  frozen  add  some  more 
and  freeze  again,  and  so  on  until  the  tissue  is  completely  covered 
and  frozen  into  a  solid  mass.  This  proportion  of  gum  and 
syrup  works  well  in  a  temperature  of  60°  F. ;  but  when  higher, 
less  syrup  is  required — when  lower,  more.  The  syrup  is  used  to 
prevent  freezing  too  hard,  so  some  judgment  must  be  exercised 
in  the  matter. 

6.  The  best  instrument  for  making  the  sections  is  the  blade  of 
a  carpenter's  plane.  Hold  it  firmly  in  the  right  hand,  and  work 
the  microtome  screw  under  the  machine  with  the  left.  Plane 
off  the  sections  as  quickly  as  possible.     They  should  all  collect 


150 


MODERN  MICROSCOPY 


on  the  plane  iron.  If  they  roll  up  or  fly  off,  the  tissue  is  frozen 
too  hard,  or  there  is  not  enough  syrup  in  the  gum.  If  the 
former  is  the  case,  allow  the  mass  to  thaw  a  little ;  if  the  latter, 
add  some  more  syrup  to  the  gum  mixture,  and  soak  the  tissue 
again. 

When  the  sections  are  cut,  place  them  in  a  saucer  of  water, 
which  must  be  changed  several  times  until  all  trace  of  gum  is 


Fig.  64.— Cathcart's  Microtome. 

removed.  Water  that  has  been  boiled  and  allowed  to  cool  will 
remove  the  gum  sooner  than  cold  water.  When  quite  free  from 
gum,  the  sections  may  be  bottled  up  in  methylated  spirit  until 
required  for  staining. 

Embedding  in  Celloidin. — Dissolve  Schering's  celloidin  in 
equal  parts  of  absolute  alcohol  and  ether  until  the  solution  is 
as  thick  as  glycerine.  Divide  the  solution  into  2  parts,  to  1 
of  which  add  an  equal  part  of  absolute  alcohol  and  ether.  De- 
hydrate the  specimen  in  absolute  alcohol  for  several  hours,  then 


EMBEDDING  TISSUES  AND  SECTION-CUTTING     151 

transfer  to  the  thinner  solution  of  celloidin,  and  soak  until 
perfectly  saturated  ;  place  in  the  thick  celloidin  for  about  an 
hour,  or  until  required.  Take  a  cork  and  paint  over  one  end  a 
layer  of  celloidin,  and  let  it  dry  ;  this  will  prevent  air -bubbles 
rising  from  the  cork  and  lodging  in  the  mass.  Take  the  specimen 
from  the  celloidin  and  lay  it  on  the  cork,  and  let  it  stand  for  a 
minute  or  two,  then  add  some  more  celloidin  until  the  tissue  is 
completely  covered,  and  set  aside,  and  when  the  mass  has  attained 
such  a  consistency  that  on  touching  it  with  the  finger  no  impres- 
sion will  remain,  place  it  in  50  per  cent,  alcohol  for  an  hour  or 
two  to  complete  the  hardening,  or  it  may  remain  there  until 
required.  The  embedded  mass  can  now  be  placed  between  two 
pieces  of  carrot,  and  put  into  an  ordinary  microtome,  and  the 
sections  made  with  a  knife  or  razor,  which  must  be  well  wetted 
with  methylated  spirit ;  or  the  embedded  specimen  can  be  re- 
moved from  the  cork,  and,  after  soaking  in  water,  it  can  be 
transferred  to  gum  and  syrup,  and  the  sections  made  with  a 
Cathcart  freezing  microtome.  If  it  is  desired  to  remove  the 
celloidin  from  the  sections,  soak  them  in  equal  parts  of  absolute 
alcohol  and  ether.  When  all  the  celloidin  is  removed,  transfer 
to  distilled  water,  then  into  the  stain.  After  staining,  wash  in 
distilled  water,  dehydrate,  clear  in  clove  oil,  and  mount  in  Canada 
balsam. 

When  it  is  not  desirable  to  remove  the  celloidin  from  the 
sections,  they  should  be  stained  in  borax  carmine  or  hematoxylin. 
The  former  stains  celloidin,  but  the  colour  is  removed  by  washing 
in  acidulated  alcohol.  Hematoxylin  only  stains  it  slightly.  All 
the  aniline  dyes  stain  it  deeply ;  they  should  not  be  used. 

Tissues  are  usually  stained  in  bulk  before  they  are  infiltrated 
with  celloidin.  When  so,  the  sections  must  be  dehydrated  in 
methylated  spirit,  cleared  in  oil  of  bergamot  or  origanum,  and 
mounted  in  Canada  balsam. 

When  desirable,  sections  infiltrated  with  celloidin  may  be 
mounted  in  Farrant's  medium  or  glycerine  jelly.  Wash  away 
all  trace  of  alcohol  with  water,  and  mount  in  either  of  the  above 
media  in  the  ordinary  way. 

Celloidin  is  an  excellent  medium  for  infiltrating  many  specimens 
of  both  animal  and  vegetable  subjects.  The  following  are  a  few 
of  these  : 


152  MODEEN  MICEOSCOPY 

Flower-buds  of  lily,  yucca,  evening  primrose,  poppy,  dandelion, 
and  anthers  ;  worms,  leech,  flukes,  gills  and  organs  of  mussels, 
heads  of  frogs,  newts,  sponges,  etc. 

For  flower-buds  proceed  as  follows  :  Harden  the  bud  in  methy- 
lated spirit  in  the  ordinary  way.     Then  take  a  piece  of  fine  silk 
or  cotton  and  tie  it  round  the  centre  of  the  bud  to  hold  the  parts 
together  ;  now  with  a  sharp  knife  cut  off  each  end  of  the  bud  so 
that  the  celloidin  may  easily  penetrate  to  the  interior.    Now  place 
the  specimen  in  equal  parts  of  absolute  alcohol  and  ether  for  at 
least  twelve  hours.   Then  transfer  to  the  thin  solution  of  celloidin, 
and  soak  until  completely  infiltrated.   Eemove  and  place  in  thick 
celloidin  for  about  twelve  hours.     Take  out  of  celloidin  on  the 
point  of  a  needle,  and  hold  exposed  to  the  air  for  a  few  minutes, 
to  dry  the  celloidin  around  the  exterior  of  the  bud.     When  dry, 
push  gently  off  the  needle  into  some  methylated  spirit,  and  soak 
for  at  least  twelve  hours  to  complete  the  hardening  of  celloidin. 
The  specimen  may  then  be  embedded  in  carrot,  and  the  sections 
may  be  cut  in  any  ordinary  well  microtome.    Worms  must  be  cut 
up  into  pieces  of  about  J  or  J  inch  long ;  these  are  then  dehy- 
drated in  equal  parts  of  ether  and  alcohol,  infiltrated  with  and 
embedded  in  celloidin,  and  then  treated  in  exactly  the  same  way 
as  directed  for  flower-buds. 

When  a  number  of  celloidin  masses  are  prepared  for  future 
use,  they  must  be  preserved  in  a  vessel  of  methylated 
spirit. 

Embedding  in  Gelatine. — This  method  is  very  useful  for 
hairs,  cotton,  silk,  wool,  and  all  such  fibres.  Take,  for  example, 
some  human  hairs  about  h  inch  long,  and  make  a  bundle  of 
them  ;  tie  them  together  either  with  a  long  hair  or  with  some 
fine  cotton.  Place  the  bundle  in  warm  water  and  soak  for  a 
few  minutes.  Now  make  up  a  strong  solution  of  some  clear 
transparent  gelatine.  Cox's  is  very  good — say  1  ounce  of  gela- 
tine to  6  of  water.  Transfer  the  bundle  of  hair  to  this,  place 
in  a  warm  water-bath,  and  soak  until  the  gelatine  has  penetrated 
all  through  the  bundle.  Eemove  from  gelatine  on  the  point  of 
a  needle,  and  allow  the  mass  to  cool ;  then  place  in  methylated 
spirit  for  about  twelve  hours.  The  embedded  mass  may  then 
be  placed  in  a  cylinder  of  carrot  and  transverse  sections  cut  in 
the  ordinary  well  microtome.     The  sections  when  cut  are  to  be 


EMBEDDING  TISSUES  AND  SECTION-CUTTING     153 

placed  in  strong  spirit  to  dehydrate ;  they  are  then  cleared  in 
clove  oil  and  mounted  in  Canada  balsam. 

Heads  of  frogs,  newts,  and  many  other  specimens  may  be 
infiltrated  and  embedded  in  gelatine,  but  they  must  all  be  stained 
in  bulk  before  they  are  infiltrated,  because  the  sections  must  not 
come  in  contact  with  water  in  any  form  ;  moreover,  if  the  sections 
were  stained  the  gelatine  would  be  coloured  as  well  as  the  tissues. 

The  Rocking  Microtome. — This  machine  is  made  by  the 
Cambridge  Scientific  Instrument  Company.  It  is  only  used  for 
specimens  infiltrated  with  paraffin,  and  it  is  automatic — that  is 
to  say,  it  can  be  set  to  cut  sections  of  definite  thickness,  and 


Fig.  65. — Rocking  Microtome. 

every  time  the  handle  is  pulled  a  section  is  cut,  and  the  speci- 
men is  moved  forward  ready  for  another. 

Infiltrate  the  tissue  with  paraffin  in  the  ordinary  way  in  a 
pill-box,  and  when  the  paraffin  has  set,  remove  the  box  and 
trim  the  paraffin  into  a  rectangular  block.  Take  care  to  keep 
the  edges  quite  parallel,  so  that  they  may  adhere  together,  as 
the  sections  are  cut  and  form  a  riband.  The  Cambridge  Instru- 
ment Company  make  an  apparatus  for  embedding,  called  embed- 
ding L's.  If  these  are  used,  perfectly  rectangular  blocks  are 
formed  ready  for  fixing  to  the  brass  cap  at  the  end  of  the  arm 
of  the  microtome,  which  is  filled  with  paraffin  ;  this  should  be 
warmed  over  a  spirit-lamp,  and  the  block  containing  the  speci- 
men is  to  be  pressed  against  the  melted  paraffin  until  it  adheres 
firmly. 


CHAPTER  IX 

STAINING  ANIMAL  SECTIONS  AND  MOUNTING  IN 

CANADA  BALSAM 

All  sections  of  organs  and  tissues  should  be  stained  with  some 
colouring  reagent,  so  that  their  structure  may  be  made  more 
apparent.  Certain  parts  of  the  tissue  have  a  special  affinity  for 
the  dyes  or  stain ;  they  therefore  become  more  deeply  tinted, 
and  stand  out  clearly  from  the  surrounding  tissues. 
The  following  staining  reagents  are  the  most  useful : 
Grenacher's Alcoholic  Borax  Carmine. — Carmine, 3 grammes; 
borax,  4  grammes ;  distilled  water,  100  c.c.  Dissolve  the  borax  in 
the  water,  add  the  carmine,  and  apply  gentle  heat  until  all  is 
dissolved ;  then  add  100  c.c.  of  70  per  cent,  alcohol,  filter,  and 
keep  in  a  stoppered  bottle. 

Staining  Process. — 1.  Place  the  section  in  distilled  water  to 
wash  away  the  alcohol,  then  place  a  little  of  the  carmine  in  a 
watch-glass,  and  immerse  the  section  for  from  three  to  five 
minutes. 

2.  Wash  the  section  in  methylated  spirit. 

3.  Take  of  methylated  spirit  5  parts,  and  of  hydrochloric 
acid  1  part,  and  mix  them  well  together.  A  quantity  of 
this  acid  solution  may  be  made  up  and  kept  ready  for  use  at 
any  time. 

Immerse  the  section  in  the  above,  and  leave  it  to  soak  for 
about  five  to  ten  minutes,  or,  if  overstained,  until  the  desired 
tint  is  obtained.  Sections  of  skin  and  scalp  may  be  left  until 
all  colour  is  removed  from  the  fibrous  tissues ;  the  glands,  hair 
follicles,  and  Malpighian  layer  will  then  stand  out  clearly. 

4.  Wash  the  section  well  in  methylated  spirit  to  remove  all 
traces  of  the  acid,  then  transfer  to  some  perfectly  clean  and 

154 


STAINING  ANIMAL  SECTIONS  155 

strong  methylated    spirit   for   from   ten    to   fifteen  minutes   to 
dehydrate. 

5.  Place  some  oil  of  cloves  in  a  watch-glass,  take  the  section 
from  the  spirit  on  a  lifter,  and  carefully  float  it  on  to  the  surface 
of  the  oil,  in  which  it  must  remain  for  about  five  minutes.  This 
process  is  called  clearing;  the  object  of  it  is  to  remove  the  alcohol 
and  to  prepare  the  section  for  the  balsam. 

6.  Transfer  the  section  to  some  filtered  turpentine  to  wash 
away  the  oil  of  cloves,  and  mount  it  in  Canada  balsam.  Sections 
may  be  mounted  in  Canada  balsam  direct  from  the  oil  of  cloves, 
but  it  is  better  to  wash  in  turpentine  first,  because  if  much 
oil  is  mixed  with  the  balsam  it  will  not  dry ;  the  oil  also 
has  a  tendency  to  cause  the  balsam  to  turn  a  dark  yellow 
colour. 

Ehrlich's  Hematoxylin. — Hematoxylin,  30  grains  ;  absolute 
alcohol,  3^  ounces ;  distilled  water,  3^  ounces  ;  glycerine,  3h 
ounces ;  and  ammonia  alum,  30  grains.  Dissolve  the  hematoxylin 
in  the  alcohol  and  the  alum  in  the  water  ;  mix  the  two  solutions 
together,  and  add  the  glycerine  and  3  drachms  of  glacial  acetic 
acid.  The  mixture  must  now  be  left  exposed  to  light  for  at 
least  a  month,  then  filter  and  keep  in  a  stoppered  bottle. 

Staining  Process. — 1.  If  the  specimen  has  been  hardened  in 
any  of  the  chromic  solutions,  place  the  section  in  a  1  per  cent, 
watery  solution  of  bicarbonate  of  soda  for  about  five  minutes, 
then  wash  well  in  distilled  water.  If  it  is  a  spirit  preparation 
the  soda  will  not  be  required,  but  all  sections  must  be  washed  in 
distilled  water  before  they  go  into  hematoxylin  stain. 

2.  To  a  watch-glassful  of  distilled  water  add  from  10  to  20 
drops  of  the  hematoxylin  solution,  and  immerse  the  section  for 
from  ten  to  thirty  minutes. 

3.  Wash  in  distilled  water,  then  in  ordinary  tap  water ; 
the  latter  will  fix  the  dye  and  cause  the  colour  to  become 
blue. 

When  a  section  has  been  overstained  with  hematoxylin,  the 
excess  of  colour  may  be  removed  by  soaking  it  for  a  few  minutes 
in  a  |  per  cent,  solution  of  glacial  acetic  acid  in  distilled  water, 
then  wash  again  in  tap  water. 

4.  Dehydrate  in  methylated  spirit. 

5.  Clear  in  clove  oil,  and  mount  in  Canada  balsam. 


156  MODEKN  MICROSCOPY 

Double  Staining  with  Hematoxylin  and  Eosin. — Stain  the 
section  in  hematoxylin,  as  directed  above,  then  place  it  in  an 
alcoholic  solution  of  eosin — about  1  grain  of  eosin  to  1  ounce 
of  methylated  spirit  is  strong  enough — and  let  it  soak  for  about 
five  minutes  ;  wash  well  in  methylated  spirit,  clear  in  clove  oil, 
and  mount  in  Canada  balsam. 

Ehrlich's  hematoxylin  is  a  good  all-round  stain,  but  as  it  is 
acid  it  must  not  be  used  for  tissues  of  a  mucous  nature,  such 
as  the  umbilical  cord,  and  many  tumours  containing  mucous  or 
gelatinous  tissues.    Delafield's  hematoxylin  should  then  be  used. 

To  400  c.c.  of  a  saturated  aqueous  solution  of  ammonia  alum 
add  4  grammes  of  hematoxylin  dissolved  in  25  c.c.  of  absolute 
alcohol ;  leave  the  solution  exposed  to  the  light  and  air  in  an 
unstoppered  bottle  for  three  or  four  days ;  filter,  and  add  to  the 
filtrate  100  c.c.  of  glycerine  and  100  c.c.  of  wood  spirit  (methylic 
alcohol) ;  allow  the  solution  to  stand  in  the  light  until  it  becomes 
of  a  dark  colour,  refilter,  and  keep  in  a  stoppered  bottle. 

Use  as  directed  for  Ehrlich's  stain. 

Aniline  Blue -Black.  —  Dissolve  30  grains  of  nigrosine  in 
Sh  ounces  of  distilled  water,  then  add  1  ounce  of  rectified  alcohol 
and  filter.  This  stain  is  only  used  for  sections  of  brain  and 
spinal  cord.  Immerse  the  sections  for  from  thirty  to  sixty 
minutes,  wash  in  water,  dehydrate  in  methylated  spirit,  clear 
in  clove  oil,  and  mount  in  Canada  balsam. 

Aniline  Blue. — Make  a  1  per  cent,  solution  of  soluble  aniline 
blue  in  distilled  water  and  filter.  Stain  the  section  for  five  to 
ten  minutes,  wash  in  water,  and  place  in  methylated  spirit,  in 
which  it  must  soak  until  the  excess  of  colour  is  removed.  Clear 
in  clove  oil  and  mount  in  Canada  balsam. 

This  stain  is  useful  for  cardiac  glands  of  the  stomach,  brain, 
and  spinal  cord. 

Golgi's  Nitrate  of  Silver  Methods. — These  are  chiefly  em- 
ployed for  investigating  the  relations  of  cells  and  fibres  in  the 
central  nervous  system.  Two  methods  are  mostly  used,  as 
follows  : 

(a)  Very  small  pieces  of  the  tissue,  which  have  been  hardened 
for  some  weeks  in  bichromate  solution  or  Miiller's  fluid,  are 
placed  for  half  an  hour  in  the  dark  in  0'75  per  cent,  nitrate  of 
silver  solution,  and  are  then  transferred  for  twenty-four  hours  or 


STAINING  ANIMAL  SECTIONS  157 

more  to  a  fresh  quantity  of  the  same  solution  (to  which  a  drop 
or  two  of  formic  acid  may  be  added).  They  may  then  be  hardened 
with  50  per  cent,  alcohol,  and  sections,  which  need  not  be  thin, 
are  cut  either  from  celloidin  with  a  microtome  or  with  the  free 
hand.  The  sections  are  mounted  in  Canada  balsam,  which  is 
allowed  to  dry  on  the  slide.  They  must  not  be  covered  with  a 
cover-glass,  but  the  balsam  must  remain  exposed  to  the  air. 

(b)  Instead  of  being  slowly  hardened  in  bichromate,  the  tissue 
is  placed  at  once  in  very  small  pieces  in  a  mixture  of  bichromate 
and  osmic  (3  parts  of  Muller's  fluid  to  1  of  osmic  acid).  In  this 
it  remains  from  two  to  five  days,  after  which  the  pieces  are  treated 
with  silver  nitrate,  as  in  the  other  case.  This  method  is  not 
only  more  rapid  than  the  other,  but  is  more  sure  in  its  results. 

Mounting  in  Canada  Balsam.  —  Take  3  ounces  of  dried 
Canada  balsam  and  dissolve  in  3  fluid  ounces  of  pure  benzol, 
filter,  and  keep  in  an  outside  stoppered  bottle.  Clear  the 
section  in  clove  oil,  and  place  in  turpentine.  Clean  a  cover- 
glass  and  a  slide,  place  a  few  drops  of  balsam  on  the  centre  of 
the  latter,  take  the  section  from  the  turpentine  on  a  lifter,  allow 
the  excess  of  turpentine  to  drain  away,  and  with  a  needle-point 
pull  the  section  off  the  lifter  into  the  balsam  on  the  slide.  Now 
take  up  the  cover-glass  with  a  pair  of  forceps,  and  bring  its  edge 
in  contact  with  the  balsam  on  the  slide  ;  ease  it  down  carefully, 
so  that  no  air-bubbles  are  enclosed,  and  with  the  points  of  the 
forceps  press  on  the  surface  of  the  cover  until  the  section  lies 
quite  flat,  and  the  excess  of  balsam  is  squeezed  out.  The  slide 
must  now  be  put  aside  for  a  clay  or  two  to  allow  the  balsam 
to  harden ;  the  exuded  medium  may  then  be  washed  away  with 
some  benzol  and  a  soft  camel's-hair  brush,  after  which  dry  the 
slide  carefully  with  a  cloth  and  apply  a  ring  of  cement.  The 
above  method  answers  well  for  mounting  sections  quickly,  but 
when  time  will  admit  the  following  is  a  much  better  way  :  Clear 
the  section  and  place  it  in  turpentine  ;  clean  a  cover-glass,  and 
moisten  the  surface  of  a  slide  with  your  breath  ;  apply  the  cover- 
glass  to  the  slide,  and  make  sure  that  it  adheres.  Place  a  few 
drops  of  balsam  on  the  cover,  into  which  put  the  section.  Now 
put  the  slide  away  in  a  box,  or  in  some  place  out  of  reach  of  dust, 
for  twelve  hours,  so  that  the  benzol  may  evaporate  from  the 
balsam.    Clean  a  slide,  warm  it  gently  over  the  flame  of  a  spirit- 


158  MODEKN  MICEOSCOPY 

lamp ;  apply  a  drop  of  balsam  to  the  surface  of  the  hardened 
balsam  on  the  cover-glass ;  take  the  cover  up  in  a  pair  of  forceps, 
and  bring  the  drop  of  fresh  balsam  in  contact  with  the  centre  of 
the  warmed  slide.  Ease  the  cover  down  carefully,  so  that  no 
air-bubbles  may  be  enclosed,  press  on  the  surface  of  the  cover- 
glass  until  the  section  lies  quite  flat ;  set  the  slide  aside  to  cool. 
The  exuded  balsam  may  then  be  washed  away  with  methylated 
spirit  and  a  soft  rag,  and  a  ring  of  cement  applied. 

Staining  in  Bulk.  —  Place  small  pieces  of  the  tissue  in 
Grenacher's  alcoholic  carmine  for  from  one  to  three  days,  then 
transfer  to  a  J  per  cent,  solution  of  hydrochloric  acid  in  methy- 
lated spirit  for  from  one  to  twelve  hours,  according  to  the  size 
of  the  tissue.  Wash  well  in  spirit,  and  soak  for  a  day  in  90  per 
cent,  spirit. 

The  specimen  may  then  be  infiltrated  and  embedded  in  paraffin, 
celloidin,  or  gelatine,  but  be  careful  to  follow  the  instructions 
previously  given  with  each  method. 

Flemming's  Method  for  Staining  Karyokinetic  Nuclei. — 
Fix  the  tissue  in  the  following  Flemming's  solution  : 

Osmic  acid,  1  per  cent,  solution  ...  ...  80  c.c. 

Chromic  acid,  10  per  cent,  solution  ...  15  c.c. 

Glacial  acetic  acid  ...         ...         ...  ...  10  c.c. 

Distilled  water        ...         ...         ...  ...  95  c.c. 

The  fixing  process  is  usually  complete  in  twelve  hours  ;  then 
wash  the  tissue  thoroughly  in  water  and  harden  in  alcohol  of 
gradually  increasing  strength.  Now  place  small  shreds  or  thin 
sections  in  a  saturated  alcoholic  solution  of  safTranin  mixed  with 
an  equal  quantity  of  aniline  water  for  two  days.  The  tissue  is 
then  to  be  washed  in  distilled  water.  It  is  then  soaked  in  abso- 
lute alcohol  until  the  colour  is  removed  from  everything  except 
the  nuclei.  It  is  then  again  rinsed  in  water  and  placed  in  a 
saturated  watery  solution  of  gentian  violet  for  two  hours,  washed 
again  in  distilled  water,  decolorized  in  alcohol  until  only  the 
nuclei  are  left  stained  ;  then  transfer  to  bergamot  oil  and  mount 
in  xylol  balsam. 

Weigert-Pal  Method  for  the  central  nervous  system,  by 
which  all  medullated  fibres  are  stained  darkly,  while  the  grey 
substance  and  any  sclerosed   tracts  of   white  matter   are    left 


STAINING  ANIMAL  SECTIONS  159 

uncoloured.  Pieces  of  brain  or  spinal  cord  which  have  been 
hardened  in  Miiller's  fluid  are  to  be  placed  direct  in  gurn-water 
and  syrup  and  soaked  for  a  few  hours ;  then  make  sections  with 
a  freezing  microtome,  and  place  them  in  water,  and  from  this 
transfer  to  Marchi's  fluid,  as  follows  : 

Miiller's  fluid        ...  ...         ..  ...     2  parts, 

Osmic  acid,  1  per  cent.    ...         ...         ...     1  part, 

and  soak  for  a  few  hours.  They  are  then  washed  in  water  and 
transferred  to  the  following  stain  :  Dissolve  1  gramme  of  hema- 
toxylin in  a  little  alcohol,  and  add  to  it  100  c.c.  of  a  2  per  cent, 
solution  of  glacial  acetic  acid,  in  which  leave  the  section  for 
twelve  hours  ;  it  will  then  be  quite  black.  Wash  again  in  water, 
and  place  in  a  J  per  cent,  solution  of  potassic  permanganate  for 
five  minutes  ;  rinse  with  water  and  transfer  to  Pal's  solution 
(sulphate  of  soda,  1  gramme ;  oxalic  acid,  1  gramme ;  distilled 
water,  200  c.c),  and  bleach  for  a  few  minutes.  When  sufficiently 
bleached  they  are  passed  through  water  into  alcohol,  cleared  in 
bergamot  oil,  and  mounted  in  Canada  balsam. 


Ehrlich's  Triple  Stain  for  Blood-Corpuscles. 

Saturated  watery  solution,  orange  '  G  '     ...  135  parts. 

methyl  green  .. .  110      „ 

,,  ,,  ,,  acid  fuchsin    ...  100      ,, 

To  the  above  add 

Glycerine       ...         ...         ...         ...  ...  100  parts. 

Absolute  alcohol       ...         ..  ...         ...  200 

Distilled  water  ...  ...  ...  ...  300 


J5 


This  solution  should  stand  for  several  weeks  to  allow  for 
sedimentation,  and  it  improves  with  age.  When  used  the  super- 
natant liquid  should  be  drawn  off  with  a  pipette  to  avoid  the 
sediment. 

The  cover-glasses  are  to  be  well  cleaned  with  alcohol,  and  the 
surface  of  one  is  touched  with  a  drop  of  fresh  blood,  and  another 
cover-glass  pressed  on  its  surface  until  the  blood  is  evenly  dis- 


160  MODERN  MICROSCOPY 

tributed.  The  covers  are  then  separated  and  allowed  to  dry. 
When  dry  they  must  be  still  further  hardened  over  a  spirit-lamp, 
or  on  a  hot  stage  made  of  sheet  copper,  and  kept  at  212°  F.  for 
from  fifteen  minutes  to  two  hours  ;  after  which  place  in  stain 
for  from  one  to  four  minutes,  wash  in  water,  dry,  and  mount  in 
Canada  balsam,  benzol,  or  xylol. 

The  eosinophil  granules  in  the  corpuscles  will  be  a  reddish 
hue,  the  neutrophile  granules  purple,  and  the  nuclei  bluish- 
green  or  blue. 


Toison's  Solution  for  Staining  White  Blood-Corpuscles. 

Methyl  violet J  grain. 

Neutral  glycerine  ...         ...         ...     1    ounce. 

Distilled  water  ...         ...         ...         ...     2  J  ounces. 

Mix  thoroughly  and  add — 

Chloride  of  sodium       ...         ...  ...   15    grains. 

Sulphate  of  sodium      ...         ...         ...     2    drachms. 

Distilled  water  .. .         ...         ...         ...     5 J  ounces. 

Filter  and  keep  in  a  stoppered  bottle.  Spread  blood  on  cover- 
glass,  dry,  and  immerse  in  stain  for  eleven  minutes.  Wash  in 
water,  dry  and  mount  in  Canada  balsam. 


Fixing  and  Staining  Sections  on  the  Slide. 

Mayer's  Albumen  Method. — White  of  egg,  50  c.c. ;  glycerine, 
50  c.c. ;  salicylate  of  soda,  1  gramme :  shake  well  together,  and  filter 
into  a  stoppered  bottle.  A  thin  layer  of  the  cement  is  spread  on 
a  slide  with  a  brush,  and  the  section  laid  on  it.  Now  warm 
gently  on  a  water-bath.  As  the  paraffin  melts  it  is  carried  away 
from  the  section  by  the  albumen.  The  section  may  now  be 
washed  with  turpentine,  benzole,  and  alcohol,  and  be  treated 
with  aqueous  or  other  stains,  without  fear  of  it  moving. 

Shellac  Method. — Make  a  solution  of  shellac  in  absolute  alcohol 
— it  should  be  about  the  thickness  of  oil — filter,  and  keep  in  a 
stoppered  bottle.     Warm  some  slides,  and  spread  over  them  a 


STAINING  ANIMAL  SECTIONS  161 

layer  of  the  cement  with  a  brush,  and  put  away  to  dry.  When 
dry  apply  a  very  thin  layer  of  creosote ;  this  will  form  a  sticky 
surface,  on  which  the  section  must  be  carefully  laid.  Now  heat 
the  slide  on  a  water-bath  for  about  fifteen  minutes  at  the  melting- 
point  of  the  paraffin  ;  this  will  allow  the  section  to  come  down 
on  the  shellac  film,  and  at  the  same  time  evaporate  the  creosote. 
Allow  the  slide  to  cool,  and  wash  away  the  paraffin  with  turpen- 
tine or  benzol.  If  the  section  has  been  stained  in  bulk,  a  drop 
or  two  of  Canada  balsam  is  added,  and  a  cover-glass  applied. 

To  Stain  a  Section  on  the  Slide. — Fix  section  on  slide  as 
directed  above.  Wash  away  the  paraffin  with  rectified  mineral 
naphtha,  follow  this  quickly  with  a  few  drops  of  methylated 
spirit,  and  then  with  some  distilled  water.  Now  apply  the  stain, 
and  place  the  slide  under  a  bell-glass  to  prevent  evaporation; 
or  the  slide  may  be  plunged  into  a  vessel  containing  the  staining 
solution.  When  sufficiently  stained,  wash  with  distilled  water, 
dehydrate  with  methylated  spirit,  drain  away  the  spirit,  and 
apply  a  drop  of  clove  oil  to  clear  the  specimen.  When  clear, 
drain  away  as  much  of  the  oil  as  possible,  add  a  drop  of  Canada 
balsam,  and  apply  the  cover -glass. 


11 


CHAPTER  X 

STAINING    BLOOD    AND    EPITHELIUM,    TEASING-OUT 

TISSUES,  AND  MOUNTING  IN  AQUEOUS  MEDIA 

STAINING    WITH    PICKOCAEMINE,    GOLD    CHLOBIDE, 

SILVEK  NITKATE,  AND  OSMIC  ACID 

Double  Staining  Nucleated  Blood-Corpuscles. 

Stain  A. — Dissolve  5  grains  of  eosin  in  J  ounce  of  distilled 
water  and  add  \  ounce  of  rectified  alcohol. 

Stain  B. — Dissolve  5  grains  of  methyl  green  in  an  ounce  of 
distilled  water. 

Place  a  drop  of  frog's  blood  on  a  slide,  and  with  the  edge  of 
another  slide  spread  it  evenly  over  the  centre  of  the  slip ;  now 
put  it  away  out  of  reach  of  dust  to  dry.  When  quite  dry,  flood 
the  slide  with  Stain  A  for  three  minutes.  Then  wash  with  water, 
and  flood  the  slide  with  Stain  B  for  five  minutes.  Wash  again 
with  water,  and  allow  the  slide  to  dry.  Apply  a  drop  or  two  of 
Canada  balsam  and  a  cover-glass. 

Blood  of  Mammals,  Non-Nucleated  Corpuscles. 

Spread  a  drop  of  blood  on  a  slide  and  let  it  dry  for  twelve 
hours,  then  stain  in  a  strong  alcoholic  solution  of  eosin  for  about 
five  minutes,  drain  away  the  eosin,  rinse  the  slide  in  methylated 
spirit,  let  it  dry,  apply  a  drop  of  Canada  balsam  and  the  cover- 
glass. 

Both  of  the  above  processes  should  be  carried  out  during 
dry  weather,  as  any  moisture  in  the  air  retards  the  drying 
of  the  corpuscles,  and  then  they  are  liable  to  change  their 
form. 

162 


STAINING  BLOOD  AND  EPITHELIUM,  ETC.       163 

Epithelium. — Kill  a  frog,  cut  off  its  head,  and  remove  the 
lower  jaw.  Open  the  abdomen  and  take  out  the  stomach,  and 
slit  it  open.  Place  the  head,  lower  jaw,  and  stomach  in  a  2  per 
cent,  solution  of  bichromate  of  potash  for  forty-eight  hours. 
Then  wash  gently  in  water  until  no  colour  comes  away  from 
the  specimens.  Now  place  all  three  portions  in  picrocarmine  for 
twenty-four  hours.  Remove  the  tissues  from  the  carmine,  and 
allow  the  stain  to  drain  away  from  them.  Take  the  lower  jaw 
and  scrape  the  tongue  for  squamous  epithelium,  and  place  the 
deposit  obtained  in  a  few  drops  of  glycerine  on  a  slide.  Take 
the  stomach,  remove  some  columnar  epithelium  from  its  internal 
surface,  and  place  it  in  some  glycerine  on  another  slide.  Then 
take  the  head  for  ciliated  epithelium,  which  will  be  found  at  the 
hinder  part  of  the  roof  of  the  mouth  ;  put  some  scrapings  from 
this  in  glycerine  on  a  slide  as  before.  Clean  a  slide  and  place  a 
drop  or  two  of  Farrant's  medium  on  its  centre ;  take  up  a  little 
of  the  epithelium  on  the  point  of  a  needle,  and  put  it  into  the 
medium.  Now  apply  a  cover-glass,  and  with  the  needle-point 
press  it  down  until  the  epithelial  cells  are  separated  and  spread 
evenly  between  the  cover  and  the  slide.  Set  the  slide  aside  for 
a  day  or  two,  so  that  the  medium  may  set.  Then  wash  away 
the  excess  of  medium  with  some  water  and  a  camel's-hair  brush, 
dry  the  slide  with  a  soft  rag,  put  it  in  a  turn-table,  and  run  on  a 
ring  of  cement. 

Portions  of  the  tongue,  trachea,  and  intestine  of  a  rabbit  or 
cat  may  be  treated  in  the  same  way. 

Endothelium. — Take  a  piece  of  the  omentum  of  any  small 
animal,  and  rinse  gently  in  distilled  water  to  remove  soluble 
matter.  Place  it  in  a  \  per  cent,  solution  of  silver  nitrate  for  ten 
minutes,  or  until  it  becomes  a  milky  white.  Wash  well  in  ordinary 
water,  and  expose  in  a  saucer  of  water  to  diffused  sunlight,  until 
it  assumes  a  brownish  colour.  Cut  out  a  small  piece  and  mount 
it  in  Farrant's  medium  or  glycerine  jelly.  In  this  specimen  only 
the  interstitial  cement  substance  will  be  seen.  To  compare  with 
it,  cut  out  a  similar  piece,  wash  it  in  distilled  water,  and  stain  it 
with  hematoxylin  for  ten  minutes  ;  wash  away  all  excess  of  stain 
with  distilled  water,  and  mount  in  Farrant's  medium  or  glycerine 
jelly.  In  this  specimen  the  nuclei  will  be  seen  stained  blue. 
Specimens   of    mesentery   showing   endothelium    may   also    be 


164  MODERN  MICROSCOPY 

mounted  in  Canada  balsam.  When  this  is  desired,  stain  the 
tissue  as  directed  above,  dehydrate  in  methylated  spirit,  clear 
in  clove  oil,  and  mount  in  Canada  balsam. 

Teasing-out  Tissues. — Take  a  very  small  piece  of  the  tissue, 
place  it  on  a  slide  in  a  few  drops  of  distilled  water,  and  with  a 
couple  of  needles  mounted  in  holders  carefully  separate  the  fibres 
from  each  other.  When  the  parts  are  sufficiently  isolated,  drain 
away  the  water,  add  a  few  drops  of  the  mounting  fluid,  and 
apply  the  cover-glass.  When  teasing  it  is  very  important  that  a 
proper  background  should  be  used  so  that  the  object  may  be 
easily  seen.  For  a  coloured  specimen,  a  piece  of  white  paper 
should  be  used,  and  a  transparent  white  tissue  will  be  seen  better 
on  a  dark  ground,  such  as  a  piece  of  black  paper  or  American 
cloth  ;  the  slide  should  be  examined  from  time  to  time  under  the 
low  power  of  the  microscope  to  ascertain  when  the  tissue  is  teased 
out  enough. 

White  Fibrous  Tissue. —Harden  some  tendons  from  a  rat's  tail 
in  methylated  spirit  for  a  week.  Then  soak  a  small  piece  in 
water  to  remove  all  trace  of  spirit,  place  it  on  a  slide  in  a  few 
drops  of  water,  and  tease  it  up  until  the  fibres  are  separated  from 
each  other.  Drain  away  the  water,  add  some  Farrant's  medium 
or  glycerine  jelly,  and  apply  a  cover-glass. 

Yellow  Elastic  Tissue. — Place  small  pieces  of  the  ligamentum 
nuchae  of  an  ox  in  chromic  acid  and  spirit  for  ten  days.  Then 
proceed  as  above. 

Striped  or  Voluntary  Muscle. — Harden  small  pieces  of  muscle 
of  a  pig  in  a  2  per  cent,  solution  of  bichromate  of  potash  for 
three  weeks,  then  transfer  to  methylated  spirit,  in  which  it  may 
remain  until  required.  Soak  a  piece  in  water  to  remove  the 
spirit,  place  a  very  small  fragment  on  a  slide  in  a  few  drops  of 
water,  and  with  a  couple  of  needles  tease  or  tear  the  tissue  up  so 
as  to  separate  the  fibres.  Drain  away  the  excess  of  water,  apply 
a  drop  or  two  of  Farrant's  medium  or  glycerine  jelly  and  a 
cover-glass. 

Non-Striped  Muscle. — Harden  a  piece  of  the  intestine  of  a  rabbit 
in  chromic  acid  and  spirit  for  ten  days.  Wash  in  water,  strip  off 
a  thin  layer  of  the  muscular  coats,  and  stain  it  in  hematoxylin. 
Wash  in  distilled  water,  and  then  soak  in  ordinary  tap-water 
until   the   colour   becomes   blue.     Clean   a   slide,  pass  a  small 


STAINING  BLOOD  AND  EPITHELIUM,  ETC.       165 

fragment  of  the  muscle  on  it  in  a  few  drops  of  water,  and  with 
needles  separate  the  fibres.  Drain  off  the  excess  of  water,  apply 
a  few  drops  of  Farrant's  medium  or  glycerine  jelly  and  a  cover- 
glass. 

Nerve-Fibres. — Dissect  out  the  sciatic  nerve  of  a  frog,  and 
stretch  it  on  a  small  piece  of  wood  as  follows  :  Take  a  match, 
make  a  slit  in  each  end  of  it,  into  which  put  the  ends  of  the 
nerve ;  now  place  it  in  a  1  per  cent,  solution  of  osmic  acid  for 
an  hour  or  two.  Wash  in  water,  tease  up  a  small  fragment  on 
a  slide,  and  apply  a  few  drops  of  Farrant's  medium  or  glycerine 
jelly  and  a  cover-glass. 

When  staining  with  gold  chloride,  solutions  from  h  per  cent. 
to  5  per  cent,  in  distilled  water  are  employed.  It  is  used  for 
staining  nerves  and  nerve-endings  ;  it  also  brings  out  the  cells 
of  the  cornea,  fibrous  connective  tissues,  and  cartilage. 

The  tissue  must  be  taken  from  the  animal  immediately  after 
death,  and  be  placed  in  the  solution  of  gold  for  from  half  an 
hour  to  an  hour ;  it  is  then  removed  to  distilled  water  for  twelve 
hours,  and  afterwards  exposed  to  the  action  of  diffuse  sunlight 
in  a  saturated  solution  of  tartaric  acid  or  formic  acid  until  it 
assumes  a  purple  colour. 

The  future  treatment  will  depend  on  the  nature  of  the  specimen. 

If  muscle  has  been  stained  for  nerve-endings,  place  a  small 
piece  on  a  slide,  tease  it  up,  and  examine  with  a  low  power  until 
you  find  a  nerve-fibre  terminating  in  an  end-plate  on  a  muscle- 
fibre,  separate  it  from  the  surrounding  fibres  as  much  as  possible, 
add  some  Farrant's  medium  or  glycerine  jelly,  and  apply  a 
cover-glass. 

If  cornea  or  cartilage,  make  vertical  and  horizontal  sections 
with  a  freezing  microtome,  and  mount  in  Farrant's  medium  or 
glycerine  jelly.  Sections  of  gold-stained  tissues  may  also  be 
mounted  in  Canada  balsam  ;  when  this  is  desired,  dehydrate  in 
strong  spirit,  clear  in  clove  oil  and  mount  in  Canada  balsam. 

There  are  many  ways  of  staining  with  gold,  but  the  above  is 
the  most  simple,  and  it  gives  very  good  results.  For  the  other 
methods  the  student  may  refer  to  the  larger  works  on  practical 
histology. 

Staining  with  Picrocarmine. — Rub  up  1  gramme  of  carmine 
with  10  c.c.  of  water,  and  3  c.c.  of  strong  liquid  ammonia ;  add 


166  MODERN  MICROSCOPY 

this  to  200  c.c.  of  a  saturated  solution  of  picric  acid  in  distilled 
water.  Leave  the  mixture  exposed  to  the  air  until  it  evaporates 
to  one-third  of  its  bulk  ;  filter,  and  keep  in  a  stoppered  bottle. 
Place  some  of  the  picrocarmine  in  a  watch-glass,  and  immerse 
the  section  for  from  half  an  hour  to  an  hour.  Remove  from  the 
stain  with  a  lifter,  and  place  the  section  on  a  slide  ;  drain  away 
as  much  of  the  excess  of  stain  as  possible,  and,  if  necessary, 
soak  up  what  remains  with  a  piece  of  filter-paper.  Then  add  a 
few  drops  of  Farrant's  medium,  and  apply  the  cover-glass. 

Picrocarmine  stained  tissues  should  never  be  washed ;  if  they 
are,  all  the  yellow  colour  will  be  removed,  and  the  specimen  will 
come  out  stained  with  carmine  only.  They  improve  by  keeping, 
and  the  staining  process  goes  on  for  several  days  after  they  are 
mounted  ;  that  is  to  say,  some  parts  give  up  the  stain,  and  others 
absorb  it.  Picrocarmine  may  be  purchased  in  crystals,  with 
which  a  2  per  cent,  solution  in  distilled  water  should  be  made. 

If  it  is  desirable  to  mount  a  picrocarmine  stained  section  in 
Canada  balsam  proceed  to  stain  as  above  ;  then  make  a  saturated 
solution  of  picric  acid  in  methylated  spirit,  filter,  and  dehydrate 
the  section  in  it ;  then  give  it  a  final  rinse  in  methylated  spirit, 
clear  in  clove  oil  and  mount  in  Canada  balsam. 

Farrant's  Medium. — Take  of  glycerine  and  a  saturated 
aqueous  solution  of  arsenious  acid  equal  parts,  and  mix  them 
well  together ;  then  add  as  much  powdered  gum  arabic  as  the 
mixture  will  take  up,  and  let  it  stand  for  six  weeks.  Filter,  and 
keep  in  an  outside  stoppered  bottle. 

The  above  is  difficult  to  make ;  it  is  better  to  obtain  it  ready 
for  use. 

Glycerine  Jelly. — Dissolve  1  ounce  of  French  gelatine  in  6 
ounces  of  distilled  water  ;  then  melt  in  a  water-bath,  and  add 
4  ounces  of  glycerine  and  a  few  drops  of  creosote  or  carbolic 
acid.  Filter  through  paper  while  warm,  and  keep  in  a  stop- 
pered bottle.  The  above  may  be  used  instead  of  Farrant's 
medium.  The  jelly  must,  of  course,  be  warmed  before  use. 
All  tissues  or  sections  must  be  well  soaked  in  water  before  they 
are  mounted  in  Farrant's  medium  or  glycerine  jelly,  so  that  all 
trace  of  alcohol  is  removed. 

Tissues  containing  much  air  should  be  soaked  in  water  that 
has  been  boiled  for  about  ten  minutes  and  allowed  to  cool. 


CHAPTER  XI 

STAINING  AND  MOUNTING  MICKO-ORGANISMS 

The  investigation  of  bacteria  may  be  carried  out  under  various 
conditions : 

(1)  In  fluids,  such  as  milk,  water,  blood,  pus,  etc.  (2)  On 
solid  media,  bread,  meat,  potatoes,  meat  jelly,  etc.,  or  in  the 
tissues  and  organs  of  animals.  In  the  former  case  a  drop  of 
fluid  is  placed  on  the  centre  of  a  cover-glass,  and  another  cover- 
glass  is  placed  on  it ;  the  two  glasses  are  then  to  be  rubbed 
together  to  spread  the  organisms  evenly  over  their  surfaces  ; 
they  are  then  separated  and  allowed  to  dry.  When  bacteria  are 
growing  on  solid  material,  scrape  off  a  small  portion,  put  on  a 
cover-glass,  and  treat  as  above ;  separate  the  covers,  and  allow 
to  dry.  When  the  cover  is  quite  dry,  take  it  up  with  a  pair 
of  forceps,  organisms  uppermost,  and  pass  two  or  three  times 
through  the  flame  of  a  spirit-lamp  ;  this  will  fix  the  albumen 
and  fasten  the  bacteria  to  the  glass. 

To  Stain  Bacteria  on  Cover-Glasses. — They  should  be 
floated  with  the  organisms  downwards  on  a  saturated  watery 
solution  of  any  of  the  following  aniline  dyes  :  Methyl  blue, 
methyl  violet,  gentian  violet,  fuchsin,  vesuvin,  or  Bismarck 
brown.  From  ten  to  fifteen  minutes  is  enough  for  the  first 
four  stains ;  vesuvin  and  Bismarck  brown  require  about  an 
hour.  When  the  staining  is  complete  wash  the  cover  in  distilled 
water.  If  the  colour  is  too  deep  wash  it  in  a  J  per  cent,  solution 
of  acetic  acid,  and  then  again  in  water ;  put  away  to  dry.  When 
quite  dry  add  a  drop  of  Canada  balsam,  and  mount  on  a  slide  in 
the  usual  way. 

When  bacteria  are  present  in  the  organs  of  animals  the 
tissues   should  be  hardened  in   methylated   spirit  for  about  a 

167 


168  MODEBN  MICKOSCOPY 

week,  and  very  thin  sections  with  a  freezing  microtome  cut 
from  them.  The  sections  may  be  stained  in  any  of  the  above 
dyes ;  then  wash  in  water,  dehydrate  in  absolute  alcohol,  clear 
in  oil  of  cedar  or  bergamot,  and  mount  in  balsam. 


Staining  Bacillus  Tuberculosis. 

Ehrlich's  Method  for  Double  Staining. — To  100  parts  of  a 
saturated  watery  solution  of  aniline  oil  add  11  parts  of  a  satur- 
ated alcoholic  solution  of  fuchsin,  and  filter.  Place  the  covers 
or  sections  in  the  stain  in  a  watch-glass,  and  warm  slowly  over 
a  spirit-lamp  until  vapour  rises.  Wash  in  water,  and  then 
immerse  for  about  a  minute  in  dilute  nitric  acid,  1  part  of  acid 
to  2  parts  water.  Wash  again  in  water,  and  stain  again  in  a 
solution  of  methyl  blue — 100  parts  of  distilled  water  to  20  parts 
of  a  saturated  solution  of  methylated  blue — in  alcohol  for  about 
twenty  minutes.  Wash  in  water,  and  in  the  case  of  sections 
dehydrate  in  absolute  alcohol,  clear  in  oil  of  cedar  or  bergamot, 
and  mount  in  balsam.  The  cover-glass  preparations  must  be 
dried ;  then  add  a  drop  or  two  of  balsam,  and  mount  as  above. 

Ziehl  Neelsen's  Method. — Fuchsin,  1  part;  5  per  cent,  watery 
solution  of  carbolic  acid,  100  parts ;  absolute  alcohol,  10  parts. 
Eemove  the  section  from  the  alcohol,  and  immerse  in  the  above 
stain  for  fifteen  minutes. 

Decolorize  in  a  5  per  cent,  watery  solution  of  sulphuric  acid, 
wash  well  in  water  to  remove  acid,  and  counter-stain  in  the 
following  for  five  minutes:  Saturated  alcoholic  solution  of  methyl 
blue,  1  c.c. ;  distilled  water,  5  c.c.  Wash  in  water,  dehydrate 
in  absolute  alcohol,  clear  in  cedar  oil,  and  mount  in  Canada 
balsam. 

Gibbe's  Double  Stain.  —  Eose  aniline  hydrochloride,  2 
grammes ;  methyl  blue,  1  gramme ;  rub  well  together  in  a 
mortar.  Then  dissolve  aniline  oil,  3  c.c,  in  15  c.c.  of  rectified 
spirit,  and  add  the  crystals  to  the  mixture ;  shake  well,  and 
when  all  are  dissolved  add  15  c.c.  of  distilled  water.  Place 
the  cover-glass  preparation  or  sections  in  the  stain  in  a  watch- 
glass,  and  warm  gently  over  a  spirit-lamp ;  then  let  them  soak 
for  four  or  five  minutes.     Wash  in  methylated  spirit  until  no 


STAINING  AND  MOUNTING  MICRO-ORGANISMS    169 

colour  will  come  away,  clear  in  oil  of  cedar,  and  mount  in 
Canada  balsam. 

Cover-glass  preparations  will  not  require  clearing  ;  they  are 
allowed  to  dry,  then  add  a  drop  of  balsam  and  mount  on  a 
slide. 

Anthrax  Bacillus. 

Lbffler's  Alkaline  Blue  Method.— To  100  parts  of  a  solution 
of  caustic  potash  (1  in  10,000)  in  distilled  water,  add  30  parts 
of  a  saturated  alcoholic  solution  of  methylene  blue.  Immerse 
the  sections  for  an  hour.  Wash  in  distilled  water,  and  then  in 
a  -|  per  cent,  solution  of  acetic  acid  in  distilled  water.  Wash 
away  the  acid  with  water,  dehydrate  in  alcohol,  clear  in  oil  of 
cedar  or  bergamot,  and  mount  in  Canada  balsam. 

Anthrax  bacilli  may  also  be  stained  by  Gram's  method. 

Gram's  Method. — Solution  A. — Saturated  alcoholic  solution 
of  gentian  violet,  11  parts  ;  saturated  watery  solution  of  aniline, 
100  parts.     Mix  well  together,  and  filter. 

Solution  B. — Iodine,  1  part ;  iodide  of  potassium,  3  parts  ; 
distilled  water,  300  parts. 

Solution  C. — Saturated  aqueous  solution  of  vesuvin. 

Take  the  section  from  alcohol,  and  place  in  Solution  A  for 
one  to  three  minutes.  Wash  in  alcohol,  and  transfer  to  Solution 
B  for  three  minutes.  Wash  in  alcohol,  and  place  in  Solution  C 
for  five  minutes.  Wash  in  distilled  water,  dehydrate,  clear  in 
oil  of  cedar  or  bergamot,  and  mount  in  Canada  balsam. 

Leprosy  Bacillus. — Stain  in  the  following  for  three  minutes  : 

Fuchsin 1  gramme 

Rectified  spirit    ...         ...         ...     20  c.c. 

Distilled  water 80  c.c. 

then  place  for  thirty  seconds  in  a  solution  consisting  of  90  per 
cent,  alcohol  10  parts,  and  nitric  acid  1  part.  Wash  in  water, 
dehydrate  in  absolute  alcohol,  clear  in  cedar  oil,  and  mount  in 
Canada  balsam. 

Diphtheria  Bacillus. — Employ  Loffler's  alkaline  blue  and 
proceed  as  for  anthrax. 


170  MODERN  MICROSCOPY 


Glanders  Bacillus. 

Kuhne's  Method.  —  Methylene  blue,  1  gramme;  absolute 
alcohol,  10  c.c.  When  all  the  blue  has  dissolved,  add  100  c.c. 
of  a  5  per  cent,  watery  solution  of  carbolic  acid. 

The  sections  are  transferred  from  alcohol  to  the  above  stain 
for  half  an  hour.  Wash  in  water  and  place  in  a  weak  solution 
of  acetic  acid  in  distilled  water  until  they  are  of  a  pale  blue 
colour ;  watch  carefully,  or  too  much  colour  may  be  removed  ; 
they  are  then  rinsed  in  lithia  water  (1  in  70)  1  c.c,  water  34  c.c, 
and  transferred  to  water.  The  sections  are  now  to  be  taken  up 
one  at  a  time  on  the  point  of  a  needle  and  dipped  into  absolute 
alcohol,  in  which  some  methylene  blue  has  been  dissolved.  Dehy- 
drate in  methylene  aniline  oil,  made  as  follows :  Rub  up  about 
10  grammes  of  methylene  blue  with  10  c.c.  of  aniline,  and  let 
the  mixture  settle.  When  dehydrated,  rinse  in  aniline,  and 
place  for  a  few  minutes  in  terebene  to  clear,  then  mount  in 
Canada  balsam. 

Schutz's  Method. — Stain  the  sections  or  cover-glass  films  in 
methylene  blue,  1  gramme  ;  rectified  spirit,  20  c.c.  ;  distilled 
water,  80  c.c,  for  several  hours.  Wash  in  a  J  per  cent,  solution 
of  acetic  acid,  dehydrate  in  absolute  alcohol,  and  clear  in  cedar 
oil,  and  mount  in  Canada  balsam. 

Syphilis  Bacillus. — Stain  by  Lusgarten's  method  as  follows  : 

Aniline  oil      ...          ...          ...          ...          ...  3  c.c. 

Distilled  water           ...         ...         100  c.c. 

Saturated  alcohol  solution  of  gentian  violet  11  c.c. 

Alcohol           10  c.c 

Sections  or  cover-glasses  are  placed  in  the  above  for  from  twelve 
to  twenty-four  hours.  They  are  then  transferred  to  absolute 
alcohol  for  a  few  minutes  ;  then  place  for  ten  seconds  in  a  1  per 
cent,  solution  of  permanganate  of  potassium,  and  wash  in  5  per 
cent,  solution  of  sulphuric  acid  to  decolorize  the  ground  tissue. 
Wash  in  water,  dehydrate  in  absolute  alcohol,  clear  in  cedar  oil, 
and  mount  in  Canada  balsam. 


STAINING  AND  MOUNTING  MICRO-ORGANISMS    171 

Bacillus  of  Enteric  Fever — Gaffky's  Method. — Sections  or 
cover-glasses  are  placed  for  twenty-four  hours  in  a  strong  solution 
freshly  made  by  adding  a  saturated  alcoholic  solution  of  methylene 
blue  to  distilled  water.  They  are  then  washed  in  distilled  water, 
dehydrated  in  absolute  alcohol,  cleared  in  terebene,  and  mounted 
in  Canada  balsam. 

Spirillum. — On  cover-glasses  these  are  easily  stained  by  any 
aniline  dye  solutions.  When  in  sections  use  the  following  stain 
for  twenty-four  hours  : 

Bismarck  brown         ...         ...         ...      1  gramme. 

Rectified  alcohol        ...         ...         ...    20  c.c. 

Distilled  wTater  ...         ...         ...    80  c.c. 

Then  wash  in  water,  dehydrate  in  absolute  alcohol,  and  mount  in 
Canada  balsam. 

Actinomycosis. 

Stain  the  sections  in  the  following  for  ten  minutes,  w-armed 
to  about  45°  C. :  Magenta,  2  parts  ;  aniline  oil,  3  parts  ;  rectified 
spirit,  20  parts  ;  distilled  water,  20  parts.  Wash  in  water. 
Place  in  a  concentrated  alcoholic  solution  of  picric  acid  for  five 
to  ten  minutes.  Wash  in  water,  dehydrate  in  alcohol,  clear  in 
clove  oil,  and  mount  in  Canada  balsam. 

Weigert's  Method. — Glacial  acetic  acid,  5  c.c.  ;  absolute 
alcohol,  20  c.c.  ;  distilled  water,  40  c.c.  ;  add  orseille  until  a 
dark  red  fluid  is  obtained.  Stain  the  sections  in  the  above  for  an 
hour  ;  rinse  quickly  in  alcohol.  Clear  in  cedar  oil  and  mount  in 
Canada  balsam. 

Haematozoa  of  Laveran. — Touch  a  drop  of  blood  with  a  per- 
fectly clean  cover-glass,  apply  another  cover-glass,  press  them 
gently  together,  then  slide  them  apart,  and  dry.  Now  stain 
in  an  alcoholic  solution  of  methylene  blue,  wash  in  water,  dry, 
and  mount  in  Canada  balsam. 

Filaria. — Specimens  are  obtained  by  pricking  the  finger  of 
the  patient  and  applying  a  drop  or  two  of  blood  to  a  glass  slide ; 
spread  evenly  with  the  aid  of  a  thin  glass  rod,  and  allow  it  to  dry. 
Now  apply  a  few  drops  of  Ehrlich's  hematoxylin,  and  stain  for 
about  five  minutes.     Wash  in  distilled  water,  then  stain  again  in 


172  MODEEN  MICKOSCOPY 

a  solution  of  eosin  in  alcohol,  rinse  in  water,  let  the  slide  stand 
up  on  end  to  drain  and  dry,  and  then  apply  a  drop  or  two  of 
Canada  balsam  on  the  cover-glass. 

Vermes. — Sections  of  specimens  such  as  Acaris,  Tsenia,  etc., 
may  be  made  in  the  following  way  :  Harden  the  worm  in  alcohol 
for  a  week  or  ten  days.  Then  cut  up  in  pieces  of  about  J  inch 
long,  and  soak  in  equal  parts  of  ether  and  alcohol  for  twelve 
hours  :  they  are  then  transferred  to  a  thin  solution  of  celloidin 
in  equal  parts  of  ether  and  alcohol,  and  must  remain  in  this 
until  perfectly  infiltrated.  Now  remove  from  thin  celloidin  and 
place  in  a  thicker  solution,  and  soak  again  for  twelve  hours. 
Remove  from  celloidin  on  the  point  of  a  needle,  and  hold  exposed 
to  the  air  for  a  minute  or  two  so  that  the  celloidin  may  dry  all 
round  the  exterior  of  the  specimen  ;  then  push  it  off  the  needle 
into  methylated  spirit,  in  which  it  should  remain  for  twelve 
hours  to  complete  the  hardening  of  celloidin  in  the  interior. 
Cut  transverse  sections,  and  stain  in  borax  carmine  for  five 
minutes.  Wash  in  methylated  spirit,  and  then  place  in  acidulated 
spirit— 1  part  hydrochloric  acid  in  5  of  methylated  spirit — for 
about  three  minutes  if  overstained,  until  the  excess  of  stain 
is  removed.  Wash  again  well  in  methylated  spirit  to  remove  all 
trace  of  acid.  Then  transfer,  for  about  one  to  two  minutes,  to 
absolute  alcohol,  clear  in  oil  of  origanum,  and  mount  in  Canada 
balsam. 

Great  care  must  be  taken  not  to  leave  the  section  in  absolute 
alcohol  for  more  than  two  minutes,  or  the  celloidin  will  be 
dissolved  and  the  section  will  fall  to  pieces. 

Heads  and  segments  of  tape-worms,  flukes,  etc.,  may  all  be 
mounted  whole.  Harden  in  methylated  spirit  for  a  few  days, 
then  stain  in  borax  carmine  for  from  one  hour  to  twenty-four 
hours  according  to  the  size  of  the  specimen.  Wash  in  methylated 
spirit,  and  soak  in  acidulated  spirit  until  the  excess  of  stain  is 
removed.  Then  place  in  water  for  a  few  minutes  to  soften  the 
tissue  a  little.  Place  the  specimen  on  a  glass  slide,  put  another 
slide  on  it,  and  press  down  carefully  until  quite  flat.  Now  bind 
the  two  slides  together  with  twine  and  place  in  a  jar  of 
methylated  spirit,  and  soak  for  at  least  twenty-four  hours. 
Then  remove  the  twine,  separate  the  slides  carefully,  and 
place   the    specimen    in    absolute    alcohol    for   ten    minutes   to 


STAINING  AND  MOUNTING  MICRO-ORGANISMS    173 

dehydrate.  Clear  in  clove  oil  for  one  hour,  and  mount  in 
Canada  balsam. 

Anchylostoma. — Harden  in  methylated  spirit  for  ten  days  ; 
then  stain  in  borax  carmine,  wash  in  methylated  spirit,  and 
place  in  acidulated  alcohol  to  remove  excess  of  colour.  Transfer 
to  water,  and  soak  until  all  trace  of  spirit  is  removed  ;  then 
mount  in  glycerine  jelly. 

Trichina  Spiralis. — Harden  muscle  with  trichina  encysted  in 
methylated  spirit.  Then  embed  in  celloidin,  make  longitudinal 
sections,  stain  in  borax  carmine,  pass  through  acidulated  spirit ; 
then  wash  in  water,  and  mount  in  glycerine  jelly. 

These  worms  may  also  be  isolated  from  the  muscle.  Tear  out 
a  piece  of  muscle  on  a  glass  slide  with  the  aid  of  a  dissecting 
microscope  or  pocket  lens,  separate  the  capsule  containing  the 
trichina  from  the  muscle  with  a  needle,  and  place  it  in  dilute 
hydrochloric  acid  until  the  capsule  is  dissolved  and  the  worm  is 
set  free ;  then  pick  it  up  with  a  fine  sable  brush,  wash  in  water, 
and  mount  in  glycerine  jelly. 


CHAPTER  XII 
INJECTION  OF   BLOODVESSELS 

Carmine  and  Gelatine  Injecting  Mass. — 

Pure  carmine      ...         60  grains. 

Liq.  ammonia  fort.         ...         ...         ...     2  drachms. 

Glacial  acetic  acid  86  minims. 

Gelatine  solution  (1  ounce  in  6  ounces 

of  water)       ...         ...         ...         ...     2  ounces. 

Water        2  ounces. 

Dissolve  the  carmine  in  the  ammonia  and  water  in  a  test-tube, 
and  mix  it  with  one-half  of  the  warm  gelatine.  Add  the  acid  to 
the  remaining  half  of  gelatine,  and  drop  it  little  by  little  into  the 
carmine  mixture,  stirring  well  all  the  time  with  a  stick  or  glass 
rod.  Filter  through  flannel,  and  add  a  few  drops  of  carbolic 
acid  to  make  the  mass  keep.  The  principle  to  be  remembered 
in  making  this  mass  is  this  :  the  carmine,  if  alkaline,  would 
diffuse  through  the  vessels  and  stain  the  tissues  around  them ; 
if  acid,  the  carmine  would  be  deposited  in  fine  granules,  which 
would  block  up  the  capillaries ;  hence  the  necessity  for  a 
neutral  fluid.  The  best  guides  are  the  colour  and  smell  of  the 
fluid.  It  should  be  a  bright  red,  and  all  trace  of  smell  of 
ammonia  must  be  removed.  The  gelatine  solution  is  made  by 
putting  1  ounce  of  gelatine  into  6  ounces  of  water  ;  it  must  then 
be  left  until  the  gelatine  becomes  quite  soft ;  then  dissolve  over 
a  water-bath. 

Prussian  or  Berlin    Blue  and    Gelatine    Mass. — Take    1J 

ounces  of  gelatine,  place  it  in  a  vessel  and  cover  it  with  water  ; 

allow  it  to  stand  until  all  the  water  is  absorbed  and  the  gelatine 

174 


INJECTION  OF  BLOODVESSELS  175 

is  quite  soft.  Then  dissolve  in  a  hot-water  bath.  Dissolve  1 
drachm  of  Prussian  or  Berlin  blue  and  1  drachm  of  oxalic  acid  in 
6  ounces  of  water,  and  gradually  mix  it  with  the  gelatine  solution, 
stirring  well  all  the  time  ;  then  filter  through  flannel. 

Watery  Solution  of  Berlin  Blue. — Dissolve  2J  drachms  of 
the  blue  in  18  ounces  of  distilled  water,  and  filter.  This  fluid  is 
useful  for  injecting  lymphatics. 

Injecting  Apparatus  Required. — An  injecting  syringe  fitted 
with  a  stop-cock,  and  several  cannulas  of  various  sizes. 

Directions  for  Injecting. — The  animal  to  be  injected  should 
be  killed  by  chloroform,  so  that  the  vessels  may  be  dilated,  and 
iujecfced  while  warm  ;  if  possible  it  should  be  placed  in  a  bath  of 
water  at  a  temperature  of  40°  C.  Expose  the  artery  of  the  parts 
to  be  injected,  clear  a  small  portion  of  it  from  the  surrounding 
tissues,  and  place  a  ligature  of  thin  twine  or  silk  round  it.  With 
sharp  scissors  make  an  oblique  slit  in  the  wall  of  the  vessel, 
insert  the  cannula,  and  tie  the  ligature  firmly  over  the  artery 
behind  the  point  of  the  cannula,  into  which  put  the  stop-cock. 
Fill  the  syringe  with  injection-fluid,  which  must  not  be  too  warm, 
and  take  care  not  to  draw  up  any  air-bubbles  ;  now  insert  the 
nozzle  of  the  syringe  into  the  stop-cock  and  force  in  a  little  fluid  ; 
remove  the  syringe,  so  that  the  air  may  escape,  insert  the  syringe 
again,  and  repeat  the  process  until  no  air-bubbles  come  out  of 
the  stop-cock.  You  may  then  proceed  slowly  with  the  injection. 
Half  an  hour  is  not  too  long  to  take  over  the  injection  of  an 
animal  of  the  size  of  a  cat.  The  completeness  of  an  injection 
may  be  judged  by  looking  at  the  vascular  parts,  such  as  the 
tongue,  eyelids,  and  lips.  When  the  injection  is  complete  shut 
the  stop-cock,  remove  the  syringe  and  cannula,  and  tie  the 
ligature  round  the  artery.  Now  place  the  animal  in  cold  water 
for  an  hour  to  set  the  injection-fluid.  When  quite  cold,  dissect 
out  the  organs,  cut  them  up  into  small  pieces,  and  place  them  in 
methylated  spirit  to  harden,  and  change  the  spirit  every  twenty- 
four  hours  for  the  first  three  days.  The  hardening  will  be 
complete  in  ten  days. 

Injection  of  Lymphatics  (Puncture  Method). — A  small  sub- 
cutaneous syringe  is  filled  with  a  watery  solution  of  Berlin  or 
Prussian  blue,  and  the  nozzle  is  thrust  into  the  pad  of  a  cat's 
foot.     The  injection  is  to  be  forced  into  the  tissues.     Then  rub 


176  MODEEN  MICROSCOPY 

the  limb  from  below  upwards.  This  will  cause  the  injection-fluid 
to  flow  along  the  lymphatics  and  find  its  way  into  the  glands  of 
the  groin. 

To  Inject  Lymph-Sinuses  of  Glands. — Force  the  nozzle  of 
a  subcutaneous  syringe  into  the  hilum  of  a  lymphatic  gland  of 
an  ox,  and  inject  a  watery  solution  of  Prussian  or  Berlin  blue 
until  the  blue  appears  on  the  surface  of  the  gland.  Then  place 
it  in  methylated  spirit  to  harden. 

When  blue  injection-fluid  is  used,  add  a  few  drops  of  acetic 
acid  to  the  spirit  while  hardening  the  tissues. 


CHAPTEK  XIII 

CUTTING,  STAINING,  AND  MOUNTING  VEGETABLE 

SECTIONS 

Stems,  leaves,  roots,  etc.,  should  be  hardened  in  methylated 
spirit  for  a  week  or  ten  days,  and  the  spirit  changed  every  twenty- 
four  hours  for  the  first  three  days.  The  stems  must  not  be  too 
old.  One,  two,  and  three  years'  growth  will  show  all  that  is 
required. 

"Wheat,  barley,  maize,  peas,  etc.,  are  usually  obtained  dry. 
They  must  be  placed  in  water  for  a  few  hours  or  until  they 
resume  their  natural  shape.  Then  lay  a  piece  of  blotting-paper 
on  a  plate,  moisten  it  with  water,  and  spread  a  layer  of  the  grains 
on  its  surface  ;  now  place  another  piece  of  wet  blotting-paper 
over  all,  and  put  in  a  warm  place  for  from  twelve  to  twenty-four 
hours,  so  that  the  embryo  may  begin  to  germinate.  Then 
remove  from  the  plate,  and  place  the  grains  in  a  bottle  of 
methylated  spirit,  which  must  be  changed  every  day  until  all 
trace  of  water  is  removed.  The  specimens  may  then  be  section- 
ized,  or  they  may  remain  in  spirit  until  required. 

Ovaries. — Gather  some  before  the  flower  opens,  and  others 
after  it  has  been  open  for  a  day.  You  will  then  have  the  ovules 
in  both  stages.  Place  them  in  methylated  spirit  and  change 
every  twenty-four  hours  for  the  first  three  days. 

Anthers. — Treat  in  exactly  the  same  way  as  ovaries,  but 
anthers  must  be  infiltrated  with  celloidin  before  the  sections  can 
be  cut.  Eemove  the  ends,  place  in  equal  parts  of  alcohol  and 
ether,  and  soak  for  twelve  hours ;  then  place  in  celloidin,  and, 
after  soaking  for  from  twelve  to  twenty-four  hours,  proceed  as 
directed  in  Chapter  VIII.  on  Section-Cutting. 

Some  specimens  after  being  in  spirit  are  too  hard  to  cut  easily. 
They  may  be  softened  by  soaking  in  warm  water.     Leaves  are 

177  12 


178  MODERN  MICROSCOPY 

often  particularly  troublesome  in  this  respect  ;  they  bend  and 
become  fixed  by  the  action  of  the  spirit,  and  will  not  then  stand 
the  slight  pressure  required  to  hold  them  firmly  between  the 
carrot  without  cracking.  When  this  happens,  soak  the  leaf  in 
warm  water  until  it  is  quite  pliable  ;  it  can  then  be  embedded  in 
carrot  without  any  risk  of  being -broken.  Stems  and  petioles  of 
many  palms  are  naturally  too  hard,  and  they  may  contain  a  large 
amount  of  silica.  They  must  be  soaked  in  water  for  a  while  ; 
then  transfer  to  liq.  potassae  for  from  one  to  twelve  hours.  Wash 
again  well  in  water  to  remove  all  trace  of  potash,  then  reharden 
in  methylated  spirit.  The  shells  of  many  stone  fruits  may  be 
softened  and  cut  by  this  method. 

Section-Cutting,  by  hand  and  with  a  microtome,  should  be 
done  in  the  same  manner  as  described  in  Chapter  VIII. 

Bleaching. — Vegetable  sections  generally  require  bleaching 
before  they  can  be  properly  stained.  Chlorinated  soda  is  used 
for  this  purpose.  Take  of  dry  chloride  of  lime,  2  ounces  ;  of 
washing  soda,  4  ounces ;  and  distilled  water,  2  pints.  Mix  the 
lime  in  one  pint  of  the  water  and  dissolve  the  soda  in  the  other. 
Mix  the  two  solutions  together,  shake  well,  and  let  the  mixture 
stand  for  twenty-four  hours.  Pour  off  the  clear  fluid,  filter,  and 
keep  in  a  stoppered  bottle  in  a  dark  place,  or  cover  the  bottle 
with  paper.  Soak  the  sections  in  distilled  water.  Pour  off  the 
water  and  add  a  quantity  of  bleaching  fluid.  Allow  this  to  act 
for  from  one  to  twelve  hours.  Wash  well  in  water,  which  must 
be  changed  several  times  to  remove  all  traces  of  soda.  The 
sections  may  now  be  stained,  or  they  may  be  preserved  in  spirit 
until  required. 

Staining  Borax  Carmine  (suitable  for  ovaries,  fruits,  etc.). — 
Pure  carmine,  1  drachm ;  liq.  ammoniae  fort.,  2  drachms.  Dis- 
solve the  carmine  in  the  ammonia,  and  12  ounces  of  a  saturated 
solution  of  borax  in  distilled  water.  Filter  and  keep  in  a  stop- 
pered bottle. 

1.  Put  some  stain  in  a  watch-glass,  and  immerse  the  section 
for  three  to  five  minutes. 

2.  Wash  well  in  methylated  spirit. 

3.  Take  of  hydrochloric  acid,  1  part ;  and  of  methylated 
spirit,  5  parts.  Mix  well  together,  and  soak  the  section  until 
the  colour  changes  to  a  bright  scarlet,  which  takes  about  five 


CUTTING  AND  MOUNTING  VEGETABLE  SECTIONS     179 

minutes.     The  acidulated  spirit  may  be  kept  ready  for  use  at 
any  time. 

4.  Wash  well  in  methylated  spirit.  Then  place  in  some  strong 
methylated  spirit,  and  soak  for  at  least  ten  minutes  to  dehydrate. 

5.  Place  the  section  on  the  surface  of  a  small  saucer  of  clove 
oil,  and  let  it  soak  until  clear. 

6.  Eemove  from  the  clove  oil  and  place  in  turpentine,  and 
then  mount  in  Canada  balsam. 

Full  instructions  for  mounting  in  Canada  balsam  are  given 
at  end  of  chanter. 

Hematoxylin. — Hematoxylin,  30  grains  ;  absolute  alcohol, 
3  J  ounces ;  distilled  water,  3J  ounces ;  glycerine,  3  ounces  ; 
ammonia  alum,  30  grains  ;  glacial  acetic  acid,  3  drachms.  Dis- 
solve the  hematoxylin  in  the  alcohol  and  the  alum  in  the 
water  ;  then  add  the  glycerine  and  acetic  acid.  Mix  the  two 
solutions  together  and  let  the  mixture  stand  for  at  least  a  month 
before  use. 

1.  Add  about  thirty  drops  of  the  above  to  an  ounce  of  distilled 
water,  and  stain  the  section  for  fifteen  to  thirty  minutes. 

2.  Wash  well  in  distilled  water,  and  then  in  ordinary  tap 
water.     This  will  fix  the  colour  and  make  it  deeper. 

3.  Dehydrate  in  strong  methylated  spirit  for  at  least  ten 
minutes. 

4.  Clear  in  clove  oil  and  mount  in  Canada  balsam. 

Double  Staining — Dalton  Smith's  Method. — Stems,  roots, 
and  leaves  : 

Green  Stain. — Acid  aniline  green   ...     2  grains. 

Distilled  water  ...     3  ounces. 

Glycerine       ...         ...     1  ounce. 

Mix  the  water  and  glycerine  together,  and  dissolve  the  green 
in  the  mixture. 


Carmine  Stain  A.- 

— Borax 

...     10  grains. 

Distilled  water 

1  ounce. 

Glycerine 

J  ounce. 

Alcohol  rect. 

1  ounce. 

mi 

Dissolve  the  borax  in  the  water,  and  add  the  glycerine  and 
alcohol. 


180  MODERN  MICROSCOPY 

Carmine  Stain  B.— Carmine        10  grains. 

Liq.  ammonise  ...     20  minims. 

Distilled  water         ...     30  minims. 

Dissolve  the  carmine  in  the  water  and  ammonia.  Mix  A  and 
B  together,  and  filter. 

1.  Place  the  section  in  green  stain  for  five  to  ten  minutes. 

2.  Wash  in  water. 

3.  Place  in  carmine  from  ten  to  fifteen  minutes. 

4.  Wash  well  in  methylated  spirit. 

5.  Dehydrate  and  clear  in  clove  oil.  Wash  in  turpentine  and 
mount  in  Canada  balsam. 

Double  Staining— M.  J.  Cole's  Method.— 

Pure  carmine    ...         ...         ...         ...     1  drachm. 

Liq.  ammonia 2  drachms. 

Dissolve  the  carmine  in  the  ammonia  and  add  12  ounces  of  a 
saturated  solution  of  borax  in  distilled  water.  Filter  through 
paper  and  keep  in  a  stoppered  bottle. 

Bleach  the  sections,  and  after  being  well  washed  with  repeated 
changes  of  water  they  are  placed  in  the  above  stain  for  five  to 
ten  minutes.  Then  wash  well  in  methylated  spirit,  and  soak  in 
acidulated  alcohol — 1  part  hydrochloric  acid  to  35  of  methylated 
spirit — until  the  excess  of  stain  is  removed ;  about  two  minutes 
is  usually  sufficient.  Wash  again  in  methylated  spirit  to  remove 
all  trace  of  acid.  Dissolve  5  grains  of  acid  aniline  green  in 
6  ounces  of  methylated  spirit,  and  filter  if  necessary.  Soak  the 
section  in  this  green  stain  for  at  least  half  an  hour;  then  just 
rinse  in  methylated  spirit,  clear  in  clove  oil,  and  mount  in 
Canada  balsam.  The  advantage  of  this  method  is  that  the 
section  can  remain  in  the  green  stain  for  any  time.  The 
writer  keeps  a  stock  of  sections  in  it  ready  for  mounting. 
Should  a  specimen  be  overstained  green,  the  excess  of  colour 
can  easily  be  removed  by  soaking  in  methylated  spirit  for  a  few 
minutes. 

Staining  with  Eosin. — Make  a  2  per  cent,  solution  of  eosin 
in  alcohol,  filter  if  necessary,  and  keep  in  a  stoppered  bottle. 
This  stain  is  used  for  showing  the  structure  of  sieve-tubes  and 


CUTTING  AND  MOUNTING  VEGETABLE  SECTIONS     181 

plates ;  it  stains  protoplasm  deeply.  Make  transverse  and  longi- 
tudinal sections  of  the  stem  of  a  vegetable  marrow,  and  immerse 
them  in  the  above  for  ten  minutes.  Then  wash  out  any  excess 
of  colour  with  methylated  spirit,  clear  in  clove  oil,  and  mount 
in  Canada  balsam. 

Staining  Hairs  on  Leaves. — Make  a  2  per  cent,  aqueous 
solution  of  soluble  aniline  blue,  and  filter.  Now  take,  for  example, 
a  young  leaf  of  Deutzia  scabia,  cut  it  into  small  pieces  of  about 
I  inch  square,  and  bleach  in  chlorinated  soda.  Then  wash  well 
in  water,  and  immerse  in  the  above  stain  for  twelve  hours,  wash 
well  in  water,  and  transfer  to  methylated  spirit,  in  which  they 
must  be  soaked  until  nearly  all  the  colour  is  removed.  Then 
soak  in  clove  oil  for  several  hours,  and  when  quite  clear  mount 
in  Canada  balsam. 

Leaves  of  eucalyptus  and  other  plants  showing  essential  oil 
glands  may  be  treated  in  the  same  way,  but  if  the  specimens 
have  been  preserved  in  spirit  they  must  be  soaked  in  water 
before  the  bleaching  process. 

Male  and  Female  Conceptacles  of  Fucus  and  other  Algae. 
— Place  the  specimens  in  methylated  spirit,  which  must  be 
changed  every  twenty-four  hours  for  the  first  three  days,  then 
let  them  soak  for  ten  days  or  until  required  for  cutting  into 
sections. 

Embed  a  conceptacle  in  carrot,  place  in  microtome,  and  make 
transverse  sections,  which  must  be  as  thin  as  possible.  While 
cutting,  keep  the  knife  well  wetted  with  methylated  spirit,  and, 
as  the  sections  are  cut,  put  them  into  spirit ;  no  water  must 
come  near  them.  "When  ready,  stain  the  sections  in  a  strong 
solution  of  acid  aniline  green  in  spirit  for  several  hours.  Then 
just  rinse  in  absolute  alcohol,  clear  in  clove  oil,  and  mount  in 
Canada  balsam. 

Transverse  and  longitudinal  sections  of  the  thallus  of  an  alga 
may  be  treated  in  the  same  way,  but  they  may  be  mounted 
without  staining,  as  the  tissues  are  coloured  naturally  brown 
and  yellow. 

Ovaries  of  Flowers. — Make  transverse  sections,  which  should 
be  as  thin  as  possible,  and  stain  them  either  in  borax  carmine 
or  hematoxylin,  then  clear  in  clove  oil,  and  mount  in  Canada 
balsam. 


182  MODERN  MICROSCOPY 

Anthers  must  be  infiltrated  with  celloidin  to  keep  the  pollen 
in  position.  Then  embed  in  carrot,  place  in  microtome,  and 
cut  transverse  sections.  Stain  in  borax  carmine,  and  after 
having  passed  through  acidulated  spirit,  wash  well  in  methy- 
lated spirit,  and  dehydrate  for  about  one  to  two  minutes  in 
absolute  alcohol ;  then  clear  in  oil  of  origanum  or  bergamot, 
and  mount  in  Canada  balsam. 

Flower  Buds. — Infiltrate  with  celloidin  as  directed  in  Chapter 
VIII.  on  Section-Cutting.  Embed  the  specimen  in  carrot,  and 
place  in  the  microtome.  Cut  transverse  sections,  stain  in  borax 
carmine,  and  pass  through  acidulated  spirit  to  remove  excess  of 
colour  ;  if  desired,  they  may  be  soaked  until  the  stain  is  removed 
from  everything  except  the  nuclei.  Wash  well  in  methylated  spirit, 
and  place  in  absolute  alcohol  for  from  one  to  two  minutes  ;  then 
clear  in  oil  of  origanum  or  bergamot,  and  mount  in  Canada 
balsam.  Great  care  must  be  taken  that  the  sections  do  not 
remain  too  long  in  absolute  alcohol ;  if  they  should,  the  celloidin 
will  dissolve,  and  the  sections  will  fall  to  pieces. 

Pollens. — Place  some  mature  anthers  in  a  large  pill-box,  and 
allow  them  to  become  perfectly  dry.  Shake  the  box  well  until 
all  the  pollen  is  set  free  ;  then  remove  the  anther  sacs  with  a 
pair  of  forceps,  and  place  the  pollen  in  a  bottle  of  turpentine  ; 
soak  for  several  days  to  remove  all  trace  of  air,  then  pour  off 
the  turpentine ;  take  up  a  little  of  the  pollen  on  the  point  of  a 
penknife,  and  place  it  in  a  few  drops  of  Canada  balsam  on  a 
cover- glass  ;  stir  up  with  a  needle  to  spread  the  grains  evenly 
over  the  cover,  and  put  away  to  dry.  When  the  balsam  has 
dried,  add  a  few  more  drops  of  balsam,  take  up  the  cover  with 
a  pair  of  forceps,  and  mount  it  on  a  warm  slide.  This  method 
of  mounting  must  always  be  employed  for  pollens,  because,  if 
they  are  put  up  in  any  other  way,  the  balsam  only  hardens  at 
the  edge  of  the  cover,  and  remains  in  a  more  or  less  fluid  state 
in  the  centre,  with  the  result  that,  if  the  slide  were  placed  on 
its  edge,  the  specimens  would  run  together  in  a  heap  at  the 
lower  side  of  the  cover. 

Pollens  may  also  be  mounted  as  opaque  objects  (see  p.  215  on 
Dry  Mounts). 

Pollens  may  also  be  stained  various  colours  by  aniline  dyes. 
Place  some  fresh  pollen  in  methylated  spirit,  and  soak  until  air 


CUTTING  AND  MOUNTING  VEGETABLE  SECTIONS     183 

and  most  of  the  colour  is  removed.  Then  pour  off  the  spirit  and 
add  a  strong  alcoholic  solution  of  some  aniline  dye  of  the  desired 
colour  ;  any  will  do  so  long  as  it  is  soluble  in  alcohol.  Soak  in 
the  dye  for  an  hour  or  two,  then  pour  off  the  stain,  just  rinse  in 
spirit,  pour  this  away,  and  add  clove  oil,  and  when  clear  pour 
off  the  oil ;  take  up  a  little  pollen  on  the  point  of  a  knife,  and 
mount  in  Canada  balsam  as  directed  for  unstained  specimens. 

Specimens  of  pollens  are  sometimes  stained  many  colours  on 
the  same  slide.  This  is  done  in  the  following  way  :  Take  some 
pollen  and  divide  it  into  equal  quantities,  each  one  of  which  is 
to  be  stained  in  a  different  dye.  Then  when  they  have  been 
cleared  by  the  clove  oil  they  are  all  mixed  together  and  mounted 
in  balsam. 

Pharmacological  Specimens.— Students  of  pharmacy  may 
desire  to  make  sections  of  the  dried  stems,  roots,  and  leaves  with 
which  they  deal.  Place  the  dry  specimen  in  water,  and  soak 
until  it  resumes  as  nearly  as  possible  its  natural  shape.  Then 
place  in  methylated  spirit,  which  must  be  changed  every  twenty- 
four  hours  for  three  days  to  remove  all  the  water.  Then  make 
sections  in  accordance  with  instructions  given  for  ordinary 
botanical  specimens. 

Powdered  Drugs. — Place  some  of  the  powder  in  methylated 
spirit,  and  soak  for  an  hour  or  two ;  then  pour  off  the  spirit, 
and  add  clove  oil ;  let  it  stand  a  little  while,  then  drain  off  the 
oil,  take  up  some  of  the  powder  on  the  point  of  a  knife,  place  it 
in  some  Canada  balsam  on  a  cover-glass,  mix  it  well  up  with  the 
balsam,  and  then  proceed  to  mount  it  as  directed  for  pollens. 

Some  specimens  may  not  be  suitable  for  mounting  in  Canada 
balsam ;  they  should  then  be  mounted  in  glycerine  jelly.  Mix 
the  powder  with  water,  and  soak  until  all  trace  of  air  is  removed ; 
allow  the  mixture  to  settle  down,  then  pour  off  the  water,  take 
up  a  small  quantity  of  the  deposit,  and  mount  in  glycerine 
jelly  as  directed  for  starches.  Sometimes  neither  glycerine 
jelly  nor  balsam  will  suit  these  specimens;  then  mount  dry. 

Make  an  opaque  cell,  place  a  little  patch  of  gum- water  to  its 
centre ;  allow  this  to  dry,  then  moisten  the  patch  of  gum  with 
your  breath.  Fill  the  cell  with  the  powder,  and  let  it  stand 
for  a  minute  or  two ;  then  shake  out  the  powder  that  has  not 
adhered,  and  apply  a  cover-glass. 


184  MODEEN  MICROSCOPY 

Mounting  in  Canada  Balsam.  —  Take  3  ounces  of  dried 
Canada  balsam  and  dissolve  in  3  fluid  ounces  of  benzole.  Filter 
and  keep  in  an  outside  stoppered  bottle. 

1.  Clean  a  cover-glass,  moisten  the  surface  of  a  slide  with  the 
breath,  apply  the  cover-glass  to  it,  and  make  sure  that  it  adheres. 

2.  Place  a  few  drops  of  balsam  on  the  cover-glass. 

3.  Take  the  section  out  of  the  turpentine  on  a  lifter,  and  put 
it  into  the  balsam  on  the  cover. 

4.  Put  away  out  of  the  reach  of  dust  for  twelve  hours,  to  allow 
the  benzine  to  evaporate  from  the  balsam. 

5.  Warm  a  slide  over  a  spirit-lamp  and  apply  a  drop  of  balsam 
to  that  on  the  cover-glass  ;  take  it  up  with  a  pair  of  forceps, 
and  bring  the  drop  of  fluid  balsam  in  contact  with  the  centre  of 
the  warmed  slide.  Ease  the  cover  down  carefully,  so  that  no 
air- bubbles  may  be  enclosed,  and  press  it  down  with  the  point 
of  the  forceps  until  the  section  lies  quite  flat  and  the  excess  of 
balsam  is  squeezed  out.  Allow  the  slide  to  cool,  and  the  excess 
of  balsam  may  then  be  washed  away  with  some  methylated  spirit 
and  a  soft  rag. 


CHAPTER  XIV 

THE  PREPARATION  OF  VEGETABLE  TISSUES  FOR 

MOUNTING  IN  GLYCERINE  JELLY,  ACETATE  OF 

COPPER  SOLUTION,  ETC. 

Epidermis  for  Stomata—  Take  a  leaf,  remove  the  edges  with 
a  pair  of  scissors,  and  then  cut  the  remainder  up  into  small 
pieces  of  about  J  inch  square.  Place  these  in  a  test-tube,  add 
nitric  acid,  and  boil  gently  over  a  spirit-lamp  for  about  a  minute, 
then  add  a  few  grains  of  chlorate  of  potash,  and  bring  to  the 
boiling-point  again.  Pour  away  the  acid  and  add  water,  which 
must  be  changed  several  times  until  all  trace  of  acid  is  removed. 
The  epidermis  will  then  be  found  quite  clean,  and  it  may  be 
stained  and  mounted  at  once,  or  be  placed  in  spirit  and  kept 
until  required. 

Another  Way. — Some  epidermal  tissues  are  very  delicate 
and  will  not  stand  the  acid  treatment.  When  this  is  the  case 
cut  the  leaf  up  as  directed  above,  place  the  pieces  in  a  jar  of 
water,  and  put  aside  for  a  week  or  two.  The  action  of  water  will 
rot  the  cellular  tissue  and  set  the  epidermis  free.  Then  wash 
well  in  water,  and  should  any  particles  of  debris  adhere,  they 
can  be  removed  by  brushing  with  a  camel's-hair  brush. 

The  epidermis  of  some  plants  will  not  stand  either  of  the  above 
processes.  When  this  is  the  case,  the  only  plan  is  to  strip  off 
a  small  piece  of  the  cuticle,  lay  it  on  a  slide,  inner  side  upper- 
most, and  with  a  scalpel  carefully  scrape  away  cellular  tissue 
that  may  be  adhering.  Then  wash  in  water,  and  proceed  with 
the  staining. 

To  Stain  the  Epidermis. — Make  a  1  per  cent,  solution  of 
methyl  aniline  violet  in  distilled  water  and  immerse  the  specimen 
for  about  five  minutes.     Then  wash  in  a  -J  per  cent,  solution  of 

185 


186  MODERN  MICROSCOPY 

glacial  acetic  acid  to  remove  excess  of  colour,  wash  away  all  trace 
of  acid  with  water,  and  mount  in  glycerine  jelly  in  the  following 

way  : 

Warm  the  jelly  carefully  in  a  water-bath  until  it  is  quite  fluid. 
Warm  a  slide,  take  up  a  little  jelly  in  a  dipping-tube,  and  place 
it  on  the  slide ;  now  take  up  the  epidermis  with  a  lifter  and  put 
into  the  jelly  on  the  slide,  being  very  careful  to  avoid  making 
any  air-bubbles.  Now  take  the  cover-glass  and  apply  it  to  the 
surface  of  the  jelly,  push  down  the  cover  with  the  point  of  a 
needle  until  it  is  quite  flat,  and  then  set  aside  to  cool.  The 
above  process  applies  to  all  specimens  that  are  to  be  mounted  in 
jelly  ;  but  when  tissues  have  been  preserved  in  spirit  they  must 
be  soaked  well  in  water  before  being  mounted. 

Annular  Vessels. — Get  some  stem  of  maize,  cut  it  into  pieces 
about  |  inch  long,  and  then  cut  again  into  thin  longitudinal 
slices  ;  place  these  in  water  until  rotten.  Now  put  some  of  the 
broken-up  material  on  a  slide  and  examine  with  a  microscope  ; 
pick  out  the  annular  vessels  on  the  point  of  a  needle,  place  them 
in  some  clean  water,  and  wash  well.  Stain  in  a  weak  watery 
solution  of  acid  green,  and  after  washing  in  water,  mount  in 
glycerine  jelly. 

Scalariform  Vessels. — Treat  pieces  of  the  rhizome  of  Pteris 
aquilina  in  exactly  the  same  way  as  stem  of  maize. 

Spiral  Vessels. — Treat  pieces  of  the  stem  of  rhubarb  in  the 
same  manner  as  annular  vessels. 

Raphides  may  be  isolated,  or  they  can  be  mounted  in  situ,  in 
the  tissues  in  which  they  occur.  For  the  former,  take  some  leaves 
of  cactus,  stem  of  rhubarb,  and  root  of  Turkey  rhubarb,  cut  them 
up  into  thin  slices  longitudinally,  and  place  them  in  a  jar  of 
water,  covered  up  to  keep  out  dust,  and  put  away  until  the 
tissue  has  become  perfectly  disintegrated.  This  will  take  several 
weeks,  and  the  process  is  more  easily  carried  out  by  keeping  the 
jar  in  a  warm  place.  When  all  the  material  has  broken  up,  stir 
well  with  a  glass  rod,  and  strain  through  a  piece  of  coarse 
muslin  into  a  shallow  vessel,  such  as  a  soup  plate  ;  stir  up  again, 
and  then  allow  to  settle  for  a  minute,  so  that  the  raphides  may 
fall  to  the  bottom  of  the  plate  ;  now  pour  away  as  much  of  the 
dirty  water  as  possible,  add  more  clean  water,  and  repeat  the 
process  until  you  have  got  rid  of  all  the  disintegrated  vegetable 


THE  PEEPAEATION  OF  VEGETABLE  TISSUES     187 

fibre.  Now  pour  the  raphides  into  a  bottle,  and  if  they  are  quite 
clean,  pour  off  the  water  and  add  methylated  spirit,  in  which 
they  may  be  preserved  until  required  for  mounting. 

To  mount  isolated  raphides,  clean  a  cover-glass,  fasten  it  to  a 
slide  with  the  aid  of  your  breath,  take  up  some  of  the  raphides 
in  a  dipping-tube,  place  them  on  the  cover-glass,  and  spread 
them  evenly  over  its  surface  with  a  needle.  Place  the  slide  out 
of  reach  of  dust  until  all  the  spirit  has  evaporated,  and  the 
raphides  are  quite  dry ;  add  a  few  drops  of  Canada  balsam,  and 
put  the  slide  away  again  for  twelve  hours  ;  then  add  a  few  drops 
more  balsam,  take  up  the  cover  with  a  pair  of  forceps,  and 
mount  it  on  a  warmed  slip.  When  the  raphides  are  very  large 
they  must  be  mounted  in  balsam  that  is  rather  thicker  than  is 
usually  used. 

Raphides  in  situ  in  Tissues. — Harden  the  stems,  roots,  or 
leaves  in  methylated  spirit,  and  make  sections  in  the  ordinary 
way;  dehydrate,  clear  in  clove  oil,  and  mount  in  Canada 
balsam. 

Raphides  in  Scale-Leaves  of  Bulbs,  such  as  Onion,  Garlic,  Lily, 
Hyacinth. — Strip  off  a  thin  portion  of  the  cuticle,  place  it  in 
methylated  spirit  for  a  few  hours,  and  when  dehydrated  clear 
in  clove  oil  and  mount  in  Canada  balsam. 

Sometimes  raphides  are  rendered  too  transparent  when 
mounted  in  balsam.  When  this  is  the  case  they  must  be 
put  up  in  glycerine  jelly  in  the  following  way  : 

Isolated  Specimens. — Pour  off  the  methylated  spirit,  and  add 
water ;  pour  off  the  water,  leaving  the  raphides  at  the  bottom 
of  the  bottle.  Clean  a  cover-glass  and  a  slide.  Place  a  few 
drops  of  warmed  glycerine  j  elly  on  the  centre  of  the  slide ;  take 
up  a  few  of  the  raphides  on  the  point  of  a  penknife,  and  place 
them  in  the  glycerine  jelly,  but  do  not  stir  them  up.  Now  apply 
the  cover-glass,  and  press  it  down  carefully  with  a  needle,  giving 
it  at  the  same  time  a  twisting  motion,  to  spread  the  raphides 
evenly  between  the  cover  and  slide.  Put  away  for  an  hour  or 
two,  scrape  off  the  excess  of  jelly  with  a  penknife,  wash  in  water, 
and  then  in  methylated  spirit,  dry  with  a  cloth,  and  apply  a  coat 
of  black  enamel.  When  raphides  in  the  tissues  are  prepared  in 
glycerine  jelly,  wash  away  all  trace  of  spirit  with  water,  and 
mount  in  glycerine  jelly  as  above. 


188  MODERN  MICROSCOPY 

Starches  (Isolated  Specimens). — If  the  tissue  is  fresh,  scrape 
the  cut  surface  with  a  knife,  and  place  the  scrapings  in  a  bottle 
of  water  ;  shake  well  and  then  strain  through  fine  muslin  into 
a  shallow  vessel ;  let  the  starch  settle,  pour  off  the  water,  and 
wash  again  with  some  clean  water  until  the  starch  is  quite  clean ; 
then  place  it  in  a  bottle,  and  when  it  has  settled  to  the  bottom, 
pour  off  the  water,  and  add  methylated  spirit. 

Dried  Specimens. — Place  in  water  until  the  tissue  swells  up, 
then,  if  the  material  is  large  enough,  it  may  be  scraped  and 
treated  as  above.  If  too  small — small  seeds,  for  instance — place 
them  in  a  mortar  in  some  water,  and  carefully  break  them  up  ; 
strain  through  muslin,  wash  with  water  until  quite  clean,  and 
preserve  in  methylated  spirit. 

Starches  may  be  mounted  in  Canada  balsam  or  glycerine  jelly. 
If  the  former  is  chosen,  spread  a  little  starch  evenly  on  a  cover- 
glass,  let  it  dry,  apply  some  Canada  balsam,  and  mount  it  in  the 
ordinary  way.  For  glycerine  jelly  pour  off  the  spirit  and  add 
water,  then  allow  the  starch  to  settle  to  the  bottom  of  the  bottle ; 
pour  away  the  water.  Place  a  few  drops  of  glycerine  jelly  on  a 
slide,  take  up  some  starch  on  a  penknife,  and  place  it  in  a  little 
heap  in  the  jelly ;  now  apply  a  cover-glass,  and  press  down  with 
a  gentle  twisting  movement  until  the  starch  is  evenly  spread. 
Let  the  jelly  set,  scrape  away  the  excess,  wash  in  water,  then  in 
spirit,  dry,  and  apply  a  coat  of  cement. 

It  is  desirable  also  to  prepare  specimens  of  starch  in  situ  in 
the  tissues.  Take,  for  example,  a  potato,  cut  it  into  small 
pieces  of  about  J  inch  square,  and  harden  them  in  methylated 
spirit.  Then  embed  in  carrot  and  cut  the  sections,  which  should 
not  be  too  thin.  Stain  in  a  1  per  cent,  solution  of  methyl  aniline 
violet,  wash  in  water,  and  mount  in  glycerine  jelly. 

In  mounting  starch  in  glycerine  jelly,  care  should  be  taken 
that  the  jelly  is  not  too  hot ;  if  it  be,  the  form  of  the  starch  will 
be  altered. 

Yeast. — Get  some  fresh  baker's  yeast,  place  a  little  of  it  in  a 
bottle  of  sugar  and  water,  and  stand  in  a  warm  place  for  twenty- 
four  hours.  Pour  off  the  sugar  water,  and  add  camphor  water. 
Make  a  cell  on  a  slide  with  black  shellac  cement,  and  let  it  dry ; 
then  apply  a  second  coat  of  cement,  and  let  this  stand  for  a  few 
minutes.     Now  take  up  some  of  the  yeast  in  a  glass  tube  and 


THE  PKEPARATION  OF  VEGETABLE  TISSUES     189 

place  a  few  drops  in  the  cell;  clean  a  cover-glass,  and  bring 
its  edge  in  contact  with  the  cement  on  one  side  of  the  cell ; 
ease  it  down  carefully,  so  that  no  air-bubbles  may  be  enclosed ; 
now  press  on  the  surface  of  the  cover  with  a  needle  until  it 
adheres  firmly  to  the  cell  all  round,  drain  off  the  excess  of 
fluid,  dry  the  slide  with  a  clean  cloth,  and  apply  a  coat  of 
cement. 

Mycetozoa  or  Myxomycetes. — Most  of  these  fungi  can  be 
mounted  in  glycerine  jelly  after  soaking  in  equal  parts  of  rectified 
spirit  and  glycerine  to  remove  the  air,  but  in  those  forms  which 
possess  lime  granules  in  the  capillitium — a  character  of  impor- 
tance in  classification — the  calcareous  matter  disappears  when 
in  glycerine  in  any  form.  When  this  is  the  case,  place  the 
specimen  in  absolute  alcohol  until  all  air  is  removed,  then 
transfer  to  clove  oil,  and  mount  in  Canada  balsam.  Some 
specimens  may,  however,  be  rendered  too  transparent  by  the 
balsam  ;  if  so,  mount  them  in  a  shallow  cell  in  some  neutral 
fluid  such  as  camphor  water. 

In  their  ripe  condition  they  may  also  be  mounted  dry  as 
opaque  objects. 

Large  fungi,  such  as  Agaricus,  should  be  hardened  in  methy- 
lated spirit  for  a  week.  Then  place  the  desired  portion  in  water, 
and  soak  to  remove  spirit,  transfer  to  gum  and  syrup,  and  when 
penetrated  with  the  gum,  freeze  and  make  the  sections  with  a 
Cathcart  microtome,  wash  away  all  trace  of  gum  with  repeated 
changes   of    warm   water,   and   mount   unstained   in   glycerine 

jelly- 

Preserving  Fluid  for  Green  Algae. — Acetate  of  copper, 
15  grains ;  camphor  water,  8  ounces ;  glacial  acetic  acid, 
20  drops ;  glycerine,  8  ounces  ;  corrosive  sublimate,  1  grain. 
Mix  well  together,  filter,  and  keep  in  a  stoppered  bottle.  The 
above  fluid  preserves  the  colour  of  chlorophyll  for  a  long  time ; 
it  may  also  be  used  as  a  mounting  fluid.  For  very  delicate 
specimens  leave  out  the  glycerine. 

The  specimens  should  be  well  washed  in  water ;  then  pour  off 
the  water,  and  add  a  quantity  of  the  copper  solution. 

To  Mount  in  the  Above. — For  example,  take  Spirogyra  as  a 
filamentous  alga.  Make  a  cell  with  some  black  cement,  and  let 
it  dry  ;  then  apply  a  second  coat  of  cement,  and  allow  this  to 


190  MODERN  MICROSCOPY 

nearly  dry.  Place  some  Spirogyra  in  the  cell,  and  with  needles 
separate  the  filaments  ;  add  a  few  drops  of  copper  solution,  and 
apply  a  cover-glass  as  directed  for  yeast. 

Protococcus. — This  can  be  obtained  by  scraping  the  bark  of 
trees.  Place  it  in  a  bottle  of  water,  and  let  it  stand  for  a  few 
hours  ;  now  add  a  little  copper  solution — this  will  kill  the  speci- 
mens, and  they  will  sink  to  the  bottom  of  the  bottle ;  pour  off 
the  water,  and  add  more  copper  solution.  Now  make  a  cell  as 
for  spirogyra  ;  take  up  some  of  the  protococcus  in  a  dipping- 
tube,  and  place  them  in  a  cell ;  wait  a  minute  for  the  forms  to 
settle  on  the  bottom  of  the  cell,  and  then  apply  a  cover-glass  ; 
drain  off  the  excess  of  fluid,  dry  the  slides  with  a  cloth,  and 
apply  a  coat  of  cement. 

Volvox,  gloeocapsa,  desmids,  etc.,  may  all  be  preserved  and 
mounted  as  above. 

Antheridia  and  Archegonia  of  Mosses. — Place  some  male 
and  female  heads  of  mosses  in  methylated  spirit  for  a  few  days, 
then  transfer  to  equal  parts  of  absolute  alcohol  and  ether,  in 
which  they  must  be  soaked  for  several  hours.  Pour  off  the 
alcohol  and  ether,  and  add  a  thin  solution  of  celloidin,  and  soak 
for  two  or  three  days ;  then  remove  the  stopper  of  the  bottle, 
and  let  the  celloidin  evaporate  to  about  half  its  original  bulk. 
Now  remove  a  specimen  from  the  celloidin,  and  hold  it  in  a  pair 
of  forceps  until  the  celloidin  sets,  then  place  it  in  methylated 
spirit  and  soak  for  an  hour  or  two  to  complete  the  hardening. 
The  embedded  specimen  may  now  be  fastened  to  a  cork  with  a 
little  celloidin,  and  longitudinal  sections  made  in  a  Cathcart 
microtome,  or  it  can  be  placed  between  two  pieces  of  carrot,  and 
the  sections  made  with  any  ordinary  well  microtome.  The 
sections  must  then  be  dehydrated  in  methylated  spirit,  cleared 
in  oil  of  bergamot,  and  mounted  in  Canada  balsam ;  or,  if  desired, 
they  may  be  soaked  in  water  to  remove  spirit,  and  be  mounted 
in  glycerine  jelly. 

Fertile  Branch  of  Chara. — Chara  is  usually  very  dirty  ;  to 
clean  it,  wash  well  in  repeated  changes  of  water,  then  in  very 
dilute  acid  for  a  few  minutes  only  ;  again  wash  in  water,  and 
preserve  in  camphor  water. 

Make  a  cell  with  shellac  cement  as  directed  above,  place  a 
fertile  branch  of  Chara  in  it ;  and  examine  under  a  dissecting 


THE  PKEPAKATION  OF  VEGETABLE  TISSUES     191 

microscope  or  lens ;  with  needles  clear  away  the  leaves  from  the 
archegonia  and  antheridia,  fill  the  cell  with  camphor  water,  and 
apply  a  cover-glass. 

When  a  deep  cell  is  required  for  a  specimen  to  be  mounted  in 
acetate  of  copper,  never  use  one  made  of  any  metal.  Vulcanite 
or  glass  cells  must  be  used.  To  one  side  of  a  cell  apply  a  coat 
of  shellac  cement  and  let  it  dry  ;  now  take  a  slide  and  warm  it 
over  a  spirit-lamp ;  take  up  the  cell  in  a  pair  of  forceps,  and 
bring  the  cemented  side  in  contact  with  the  centre  of  the  warm 
slide,  and  press  it  down  until  it  adheres  firmly  ;  then  add  another 
coat  of  cement  to  the  upper  side  of  the  cell,  and  let  it  nearly  dry, 
put  in  the  specimen,  fill  the  cell  with  solution,  and  apply  the 
cover-glass. 

Prothallus  of  Fern. — Preserve  in  acetate  of  copper  and  mount 
in  the  same  fluid  in  a  shallow  cell. 

Sporangia  and  Spores  of  Fern. — Place  leaves  of  a  fern 
with  sporangia  in  methylated  spirit  for  a  few  days  to  remove  the 
air.  Then  soak  in  water  for  several  hours.  Warm  a  slide,  and 
place  a  few  drops  of  glycerine  jelly  on  its  surface,  scrape  off  some 
sporangia,  and  place  them  in  the  jelly  ;  now  apply  the  cover-glass 
very  carefully  to  avoid  scattering  the  sporangia.  The  object  is 
to  keep  them  in  a  heap  in  the  centre  until  the  cover  is  flat ;  then 
press  on  the  surface  of  the  cover  with  the  points  of  the  forceps, 
and,  if  possible,  give  the  cover  a  little  twisting  motion.  This  will 
spread  the  specimens  ;  it  will  also  rupture  some  of  the  sporangia 
and  let  out  the  spores. 

Isolating  Antheridia  and  Oogonia  from  Fucus.  —  Take 
some  conceptacles  that  have  been  hardened  in  methylated 
spirit,  and  make  thick  sections  by  hand  only  with  a  sharp 
knife.  Place  these  in  a  strong  solution  of  acid  aniline  green  in 
spirit,  and  let  them  stand  for  two  or  three  hours.  Now  place 
in  water  for  a  few  minutes,  and  they  will  at  once  swell  up  like 
a  mass  of  mucus.  Place  this  on  a  slide  and  put  another  slide 
on  top  of  it,  press  down  the  upper  slide — this  will  squeeze 
out  the  contents  of  the  conceptacles  in  little  round  masses. 
Separate  the  glasses,  pick  up  one  of  the  little  lumps  of  antheridia 
or  oogonia,  place  it  in  a  few  drops  of  glycerine  jelly  on  a  slide, 
then  apply  the  cover-glass,  which  must  be  pressed  down  to 
spread  the  specimens. 


192  MODERN  MICROSCOPY 

Digestive  Glands  in  Pitcher  Plant. — Harden  some  strips  of 
a  pitcher  in  methylated  spirit  for  a  week.  Then  place  in  water 
and  soak  for  a  few  hours.  Then  lay  the  tissue  with  the  glandular 
surface  next  to  the  glass,  and  with  a  scalpel  scrape  away  the 
outer  wall.  Now  bleach  the  glandular  portion  in  chlorinated  soda, 
then  wash  well  with  water,  stain  in  aqueous  solution  of  acid 
aniline  green,  wash  again  in  water  to  remove  excess  of  colour, 
soak  for  several  hours  in  dilute  glycerine,  and  mount  in 
glycerine  jelly. 

Aleurone. — Take  the  endosperm  of  a  castor-oil  seed,  embed  in 
carrot,  place  in  microtome,  and  cut  sections  as  thin  as  possible 
with  a  knife  wetted  with  a  little  olive  oil.  As  the  sections  are 
cut,  put  them  on  a  slide,  and  place  out  of  reach  of  dust  until  you 
are  ready  to  mount  them. 

Make  a  shallow  cell  as  directed  with  black  enamel  and  let  it 
dry,  then  proceed  as  directed  for  acetate  of  copper  mounting,  but 
use  castor  oil  instead  of  copper  solution.  When  the  cover  has 
become  fixed,  wash  away  the  exuded  oil  with  a  soft  brush  and 
some  turpentine,  and,  when  dry,  apply  a  good  finishing  coat  of 
black  enamel.  Water  and  spirit  are  apt  to  injure  the  aleurone 
grains,  so  they  should  be  avoided. 

Marine  Algae. — The  best  place  for  collecting  specimens  is  a 
rocky  shore,  and  the  most  suitable  time  is  when  the  tide  is  at  its 
lowest.  As  a  rule,  the  inshore  weeds  near  high-water  mark  are 
green,  lower  down  there  is  usually  a  belt  of  olive  forms  sheltering 
red  plants  beneath  them,  and  where  rocks  overhang  small  shallow 
pools  red  forms  also  occur  at  this  level.  At  extreme  low- water 
mark  and  beyond  it  are  found  brown  tangles  sheltering  red  plants 
again,  while  at  the  lowest  depths  the  red  weeds  occur  without 
shelter.  The  specimens  will  be  found  by  searching  the  rocks 
and  pools,  some  will  be  growing  on  pebbles  and  on  shells,  others 
will  be  attached  to  rocks,  and  varieties  may  be  found  stranded 
on  the  shore,  thrown  there  by  waves,  particularly  after  a  storm, 
the  tufts  having  been  torn  away  and  carried  inshore  from 
inaccessible  regions. 

For  collecting,  small  tin  boxes  or  an  ordinary  sponge  bag  will 
be  found  most  suitable.  A  strong  chisel  mounted  on  a  stout 
stick  will  also  be  required  for  removing  specimens  from  rocks 
that  are  out  of  reach. 


THE  PKEPAKATION  OF  VEGETABLE  TISSUES     193 

Many  specimens  may  be  preserved  in  sea  water  for  a  consider- 
able time,  but,  as  a  rule,  the  sooner  they  are  mounted  the 
better. 

Mounting"  Process. — Remove  the  specimen  from  sea  water 
and  wash  well  in  fresh  water.  Place  in  a  shallow  white  dish  or 
saucer,  select  and  cut  off  the  portion  that  is  to  be  mounted,  and 
place  it  on  a  slide  slightly  warmed,  drain  away  as  much  water  as 
possible,  and  apply  some  glycerine  jelly ;  then,  if  necessary,  lay 
or  spread  out  the  leaves  or  filaments  with  a  needle  and  apply 
the  cover-glass,  allow  the  slide  to  cool,  remove  the  excess  of 
jelly  around  the  edge  of  the  cover,  wash  the  slide  in  water,  dry, 
and  add  several  coats  of  enamel  or  varnish. 

Corallines,  whose  tissues  are  hard  and  opaque,  may  be  cleaned 
by  soaking  for  a  short  time  in  a  weak  solution  of  hydrochloric 
acid,  then  wash  well  in  water,  and  mount  in  glycerine  jelly. 


13 


CHAPTER  XV 

CUTTING,  GKINDING,  AND  MOUNTING  SECTIONS  OF 
HAKD  TISSUES— PREPAKING  METAL  SPECIMENS 

Bone. — Take  the  femur  of  a  sheep,  remove  as  much  of  the 
muscle  as  possible,  and  macerate  in  water  until  quite  clean,  then 
allow  it  to  dry. 

1.  With  a  fine  saw  make  transverse  and  longitudinal  sections. 

2.  Take  a  hone  (water  of  Ayr  stone),  moisten  it  with  water,  and 
rub  one  side  of  the  section  upon  it  until  it  is  quite  flat  and  smooth. 

3.  Wash  in  water,  and  set  aside  until  quite  dry. 

4.  Take  some  dried  Canada  balsam,  place  a  piece  on  a  square 
glass,  and  warm  gently  over  a  lamp  until  the  balsam  melts  ; 
allow  it  to  cool  a  little,  and  then  press  the  smooth  side  of  the 
section  into  it,  and  set  aside  until  cold. 

5.  With  a  fine  file  rub  the  section  down  as  thinly  as  possible. 

6.  Take  the  hone  again  and  grind  the  section  down  until  thin 
enough,  using  plenty  of  water. 

7.  Place  it,  with  the  glass,  in  methylated  spirit  until  the  section 
comes  away  from  the  glass,  then  wash  well  in  clean  water  and 
allow  to  dry. 

8.  Place  the  section  on  a  slide,  and  apply  a  very  thin  coat  of 
gum  water  to  its  upper  surface,  taking  great  care  that  the  gum 
does  not  run  under  the  section,  and  let  it  dry.  This  coat  of 
gum  will  hide  any  fine  scratches  that  may  be  left  on  the  section. 
Now  take  a  thin  cell  just  deep  enough  for  the  section,  and  apply 
a  coat  of  cement  to  its  upper  edge ;  place  the  section  in  its 
centre  with  the  gummed  side  uppermost,  and  apply  the  cover- 
glass,  which  should  come  down  on  the  bone  to  keep  it  in  the 
centre,  hence  the  necessity  of  a  cell  of  just  the  proper  depth. 
The  object  of  a  section  of  dry  bone  is  to  show  the  canaliculi ; 

194 


CUTTING  AND  MOUNTING  HARD  TISSUES        105 

when  mounted  in  fluid  of  any  kind  these  are  obliterated. 
Sections  of  teeth  are  made  in  the  same  way. 

Rock  Sections. —  Small  pieces  or  slices  of  rock  are  to  be 
ground  on  a  zinc  plate  with  the  aid  of  emery  powder  and  water 
until  one  side  is  quite  flat  and  smooth.  Then  fasten  the  polished 
surface  to  a  square  of  glass  with  some  dried  Canada  balsam,  as 
directed  for  bone,  and  allow  it  to  cool.  Grind  the  other  side  on 
the  zinc  plate  with  coarse  emery  and  plenty  of  water.  When 
moderately  thin,  take  a  piece  of  plate-glass  and  some  fine  flour- 
emery,  and  rub  the  section  down  as  thinly  as  possible.  When 
thin  enough,  wash  well  in  water  and  dry  ;  then  warm  over  a  spirit- 
lamp,  and  with  a  needle  push  the  section  off  the  glass  into  a 
saucer  of  benzole  or  turpentine,  and  allow  it  to  soak  until  all  the 
balsam  is  dissolved.  Wash  again  in  some  clean  benzole,  and 
mount  in  Canada  balsam  in  the  usual  way.  Sections  of  echinus 
spines,  shells,  and  stones  of  fruit  are  prepared  in  the  same  way 
as  bones  and  teeth  ;  but  when  the  grinding  is  finished,  the 
sections  are  to  be  passed  through  alcohol  into  clove  oil,  then 
mount  in  Canada  balsam  in  the  usual  way. 

Sections  of  coal  containing  fossils,  limestone,  spines  of  echinus, 
and  other  friable  specimens  should  be  cut  with  a  very  fine  saw, 
and  then  soaked  in  benzole  for  several  hours.  When  the  benzole 
has  saturated  the  tissue,  transfer  to  ordinary  solution  of  Canada 
balsam  in  benzole,  and  soak  again  until  the  balsam  has  penetrated 
to  the  centre.  Take  a  3x1  inch  slide,  place  the  section  on  its 
centre,  and  add  sufficient  balsam  to  cover  it.  Put  away  out  of 
reach  of  dust  until  the  benzole  has  evaporated  from  the  balsam. 
Then  place  on  a  hot  plate,  apply  gentle  heat  with  a  spirit-lamp, 
and  bake  until  the  balsam  is  quite  hard.  Grind  down  to  the 
required  thinness  on  a  hone.  Wash  well  with  water,  dry,  add  a 
few  drops  of  fluid  balsam  in  benzole,  and  apply  a  cover-glass. 

Metal  Specimens. 

The  preparation  of  specimens  of  metal  for  the  microscope 
involves  the  greatest  care,  the  principal  object  being  to  obtain  a 
perfectly  level  surface,  free  from  all  scratches  and  marks,  with 
the  highest  degree  of  polish.  This  will  be  better  illustrated  by 
an  example. 

The  student  having  obtained  a  sample  of  metal,  the  first  thing 


196  MODERN  MICROSCOPY 

to  do  is  to  carefully  file  or  grind  the  surfaces  he  wishes  to  examine. 
The  marks  thus  made  must  be  taken  out  with  a  very  smooth  file 
or  emery  cloth,  gradually  diminishing  the  coarseness  of  the  cloth 
until  he  reaches  the  finest  grade  of  all. 

From  this  stage  the  polishing  must  be  done  on  parchment  or 
chamois  leather  stretched  very  tightly  on  wood,  the  leather  being 
covered  with  fine  crocus  powder  or  rouge  moistened  with  a  little 
water. 

This  is  the  most  important  stage  of  the  specimen,  especially 
if  the  metal  be  very  soft,  and  the  student  should  frequently 
examine  the  metal  through  the  microscope — a  matter  of  a  few 
moments  only — by  clamping  it  in  the  new  metal-holder  recently 
introduced  by  Messrs.  Watson,  as  shown  on  p.  41. 

It  will  then  be  seen  that  parts  stand  in  very  high  relief.  The 
object  of  the  leather  polishing  being  to  gradually  grind  away  the 
soft  and  leave  the  hard  parts,  great  care  must  be  exercised  in 
doing  this. 

The  specimen  is  now  ready  for  further  treatment — viz.,  etching. 
The  object  of  this  is  to  further  develop  the  structure,  as  will  be 
seen  from  below. 

Etching. — This  is  done  by  various  reagents,  the  choice  of 
which  is  mainly  a  matter  of  personal  opinion,  but  perhaps  the 
most  generally  used,  and  the  best  for  beginners,  are  infusion  of 
liquorice  root  and  tincture  of  iodine.  Very  dilute  nitric  acid  and 
sulphuric  acid  are  also  used,  but  until  the  student  has  become 
thoroughly  acquainted  with  the  effects  of  the  above  he  is  not 
advised  to  use  them. 

Before  proceeding  further  it  is  advisable  to  give  an  outline 
respecting  the  effects  of  the  reagents  ;  also  the  construction  of  the 
metal. 

Steel  is  viewed  as  if  it  were  a  rock  with  various  constituents 
in  it.  There  are  three  principal  ones — viz.,  ferrite,  cementite, 
and  pearlite  (or  sorbite). 

Ferrite. — This  is  iron  free  from  carbon.  It  retains  a  very 
dull  polish,  and  is  not  stained  by  iodine  or  liquorice. 

To  develop  the  crystalline  structure  of  ferrite  a  very  dilute 
solution  of  nitric  acid  in  alcohol  should  be  used. 

Cementite. — This  is  a  very  hard  substance,  and  stands  in  relief 
after  polishing,  as  above.     It  is  very  rarely  found  in  low  carbon 


CUTTING  AND  MOUNTING  HARD  TISSUES        11)7 

steels,  and  is  left  bright  after  the  polished  surface  is  attacked  by 
iodine. 

Pearlite. — This  is  a  very  intimate  mixture  of  ferrite  and 
cementite.  If  the  steel  has  been  allowed  to  cool  slowly  from  a 
very  high  temperature,  pearlite  assumes  a  well-defined  lamellar 
structure  ;  on  the  contrary,  if  the  metal  has  been  forged  or 
reheated  at  a  very  low  temperature,  pearlite  assumes  a  granular 
appearance.     It  is  readily  acted  upon  by  iodine  or  liquorice. 

From  this  it  will  be  seen  that  steel  is  made  up  of  (1)  ferrite 
and  pearlite,  (2)  pearlite,  (3)  pearlite  and  cementite.  Other 
constituents  are  found  in  steel  after  it  has  undergone  certain 
treatment,  but  enough  has  been  said  to  guide  the  student  to 
make  a  commencement. 

The  reagent  can  now  be  applied.  This  is  done  by  either 
coating  the  specimen  with  some  protective  varnish,  leaving  the 
surface  free  that  is  to  be  acted  upon,  and  immersing  the  whole 
in  a  bath,  or  a  few  drops  may  be  applied  to  the  surface,  and  then 
carefully  spread  by  means  of  a  glass  dipping-rod. 

The  solution  should  be  allowed  to  act  for,  say,  twenty  seconds, 
then  carefully  washed  in  alcohol  or  methylated  spirit,  gently 
rubbing  the  surface  with  the  little  finger,  finally  washing  in 
water  and  drying  with  a  very  soft  piece  of  linen. 

The  metal  is  now  examined  under  the  microscope,  and  it  will 
then  be  seen  if  the  etching  has  been  sufficient ;  if  not,  it  should 
be  repeated,  as  above,  for  another  twenty  seconds.  The  student 
should  do  this  several  times,  noting  the  effect  of  the  reagent  each 
time  until  he  becomes  thoroughly  acquainted  with  its  properties. 

So  far  we  have  only  dealt  with  steel,  but  alloys  of  tin,  copper, 
etc.,  are  treated  in  exactly  the  same  way,  with  the  exception  that 
liquorice  and  iodine  are  not  used.  The  various  acids,  ammonia, 
and  caustic  potash  are  then  used  in  weak  solution  as  etching 
reagents. 

With  respect  to  the  mounting  of  the  specimens,  it  will  be  seen 
that  the  new  holder  does  away  entirely  with  the  glass  slide.  It 
often  happens  that  the  lower  edge  of  the  metal  is  left  jagged, 
and  may  also  be  broken  off  at  a  very  sharp  angle,  necessitating 
a  long  delay  in  filing  or  grinding  ;  also  the  metal  must  not  be  too 
thick  if  a  glass  slide  is  used ;  but,  as  will  be  seen,  this  labour  is 
greatly  minimized  if  the  holder  be  used. 


CHAPTEK  XVI 

PKEPAEING  AND  MOUNTING  ENTOMOLOGICAL 
SPECIMENS  FOE  THE  MICROSCOPE 

Insects  should  be  killed  with  chloroform.  They  are  then  to  be 
placed  in  methylated  spirit,  in  which  they  may  remain  until 
required  for  mounting. 

To  Prepare  a  Whole  Insect  for  Mounting  with  Pressure 
in  Canada  Balsam. — 1.  Transfer  from  methylated  spirit  to 
water,  and  let  it  soak  for  three  or  four  hours  to  remove  spirit. 

2.  Place  in  liq.  potassae — 10  per  cent,  of  caustic  potash  in  dis- 
tilled water — until  soft.  Some  specimens  will  only  require  a  few 
hours  in  the  potash,  others  need  days,  and  some  even  weeks,  to 
soften.  In  all  cases  they  must  be  carefully  watched  and  the 
action  of  the  potash  tested.  This  can  be  ascertained  by  pressing 
on  the  thorax  or  chest  of  the  insect  with  some  blunt  instrument 
such  as  the  head  of  a  pair  of  curved-pointed  forceps. 

3.  When  soft  enough,  pour  away  the  potash  and  add  water, 
which  must  be  changed  several  times  until  all  the  potash  is 
washed  away. 

4.  Pour  away  the  water  and  add  concentrated  acetic  acid,  and 
soak  for  twelve  hours,  or  until  it  is  convenient  to  go  on  with  the 
work. 

5.  Transfer  from  acetic  acid  to  water,  and  soak  for  about  half 
an  hour ;  then  place  in  a  shallow  saucer  full  of  water,  and  with 
the  aid  of  a  needle  and  a  camel' s-hair  brush  spread  out  the 
wings,  legs,  etc.  Now  take  a  slide  and  place  it  in  the  water 
under  the  insect,  lift  the  slide  up  carefully  so  that  the  insect 
may  be  stranded  on  .the  surface  of  the  slide  with  all  its  parts 
expanded.  Drain  off  the  excess  of  water,  and  lay  the  slide  down 
on  a  piece  of  white  paper,  and  with  the  aid  of  needles  or  brushes 

198 


PEEPAKING  ENTOMOLOGICAL  SPECIMENS       199 

carefully  place  all  the  limbs,  wings,  antennae,  etc.,  in  their  natural 
positions.  Now  put  a  narrow  slip  of  paper  on  each  side  of  the 
insect,  and  carefully  lay  another  slide  over  it,  press  it  down  until 
the  insect  is  squeezed  quite  flat,  tie  the  two  slides  together  with 
a  piece  of  twine,  and  place  them  in  a  jar  of  methylated  spirit  for 
at  least  twelve  hours,  or  until  required. 

6.  Eemove  the  glasses  from  the  spirit,  carefully  separate  them, 
and  with  a  soft  camel' s-hair  brush  push  the  insect  off  the  glass 
into  a  saucer  of  spirit,  and  soak  for  half  an  hour. 

7.  Take  the  insect  up  on  a  lifter,  and  float  it  on  to  the  surface 
of  a  small  saucer  of  clove  oil,  and  allow  it  to  soak  until  perfectly 
clear. 

8.  Eemove  from  clove  oil  and  place  in  turpentine  for  a  few 
minutes. 

9.  Mount  in  Canada  balsam  as  directed  for  animal  and  botani- 
cal sections. 

To  Mount  an  Insect  in  Canada  Balsam  without  Pressure. 
— Treat  with  potash  as  above,  wash  in  water,  and  place  in  acetic 
acid.  Wash  away  the  acid  with  water,  and  transfer  to  a  shallow 
saucer  of  methylated  spirit.  Take  two  needles  and  lay  out  the 
various  parts  as  quickly  as  possible  ;  if  any  parts  are  troublesome, 
hold  them  in  position  until  the  spirit  has  fixed  them.  Now  let 
it  soak  for  an  hour,  or  until  required.  Remove  from  spirit,  place 
in  clove  oil,  and  when  clear,  place  in  turpentine. 

Take  a  tin  cell  just  deep  enough  for  the  specimen,  and  apply 
a  coat  of  black  shellac  cement  to  one  side  of  it.  Allow  this  to 
nearly  dry.  Clean  and  warm  a  slide  over  a  spirit-lamp ;  take 
up  the  cell  in  a  pair  of  forceps,  and  bring  the  cemented  side  in 
contact  with  the  centre  of  the  warmed  slide ;  press  on  the  upper 
side  of  the  cell,  until  it  adheres  firmly  to  the  slide,  and  put  it 
away  to  dry.  Fill  the  cell  with  Canada  balsam,  and  see  that  it 
also  flows  over  the  upper  edge  of  the  cell,  so  that  it  may  serve 
as  a  cement  to  fasten  on  the  cover.  Take  the  insect  from  the 
turpentine  on  a  lifter,  put  it  in  the  cell,  and  with  needles  re- 
arrange the  parts  if  necessary.  Put  away  out  of  reach  of  dust 
for  twelve  hours  to  harden  the  balsam.  Place  a  drop  of  balsam 
on  one  side  of  the  cell.  Clean  a  cover-glass  of  the  same  size  as 
the  cell,  take  it  up  in  a  pair  of  forceps,  and  warm  it  gently  over 
a  spirit-lamp,  and   bring  its  edge  in  contact  with  rthe  drop  of 


200  MODERN  MICROSCOPY 

fresh  balsam  ;  ease  down  carefully,  so  as  to  avoid  air-bubbles, 
and  press  on  surface  of  cover  with  a  needle  until  it  rests  on  the 
cell  all  round.  Now  take  a  soft  brush  and  some  benzole  and 
wash  away  the  exuded  balsam  ;  dry  with  a  clean  rag,  and  apply 
a  ring  of  cement. 

To  Mount  an  Insect  in  Glycerine  without  Pressure. — 
Many  small,  soft  insects  and  their  larvae  may  be  mounted  in 
glycerine  while  fresh.  The  larger  and  harder  kinds  must  be 
soaked  in  potash  to  render  them  transparent.  Make  a  cell  of 
the  required  size,  and  fasten  it  to  a  slide  with  black  shellac 
cement,  as  directed  for  balsam  mounts.  Apply  a  coat  of  cement 
to  the  upper  side  of  the  cell,  and  allow  it  to  nearly  dry.  Fill 
the  cell  with  glycerine,  and  put  the  insect  into  it ;  spread  out 
the  wings,  legs,  etc.  Clean  and  warm  a  cover-glass,  and  apply 
its  edge  to  the  cell ;  press  down,  and  be  sure  that  it  adheres  to 
the  cement  all  round.  Wash  away  the  excess  of  glycerine  with 
some  water,  and  dry  the  slide  with  a  soft  cloth.  When  quite 
dry,  apply  a  ring  of  cement,  and  when  this  has  dried,  add 
another  coat  of  black  shellac  cement. 

The  processes  described  only  refer  to  the  study  of  the 
external  parts  of  insects ;  all  the  soft  tissues  and  internal 
organs  will,  of  course,  have  been  destroyed  by  the  potash.  Soft 
internal  organs  must  be  dissected  out  of  the  specimen  while 
under  water. 

Procure  a  guttapercha  dissecting-dish,  lay  the  insect  in  it,  and 
secure  with  pins  in  the  desired  position.  If  the  abdominal  or 
thoracic  viscera  are  required,  lay  the  insect  on  its  back ;  if  the 
nervous  system,  on  its  ventral  surface.  Fill  the  dish  with  water, 
and  with  a  pair  of  sharp-pointed  scissors  cut  through  the  chitin- 
ous  skin  on  each  side  of  the  abdomen,  taking  care  not  to  cut  too 
deeply  so  as  to  injure  the  internal  organs;  then  with  a  pair  of 
forceps  raise  and  remove  the  skin.  The  organs  may  now  be 
removed  with  the  aid  of  a  pocket-lens,  and  washed  in  distilled 
water ;  then  stain  in  borax  carmine  for  several  minutes,  wash 
in  methylated  spirit ;  then  immerse  in  acidulated  alcohol  for  a 
few  minutes,  dehydrate,  clear  in  clove  oil,  and  mount  in  Canada 
balsam. 

If  desirable  to  mount  the  specimen  in  glycerine,  stain  as  above, 
then  wash  away  all  trace  of   spirit  with  water,  and  mount  in 


PKEPAEING  ENTOMOLOGICAL  SPECIMENS       201 

glycerine  jelly ;  if  the  specimen  requires  a  cell,  it  must  be 
mounted  in  glycerine. 

Salivary  glands  of  cockroaches  and  crickets,  gizzards  of  beetles, 
and  stings  of  bees  and  wasps,  may  be  easily  removed  in  the 
following  way :  Place  the  specimen  whole  and  while  quite  fresh 
in  water,  cover  with  a  piece  of  paper  or  anything  to  keep  out 
dust,  and  let  them  soak  for  several  days  until  the  smell  becomes 
rather  unpleasant ;  then  wash  in  clean  water,  hold  the  insect 
between  the  fingers,  and  with  a  pair  of  forceps  carefully  pull  off 
the  head,  which  should  bring  with  it  the  oesophagus,  salivary 
glands,  and  stomach.  For  stings  of  wasps  and  bees  proceed  as 
follows  :  Gently  squeeze  the  abdomen  of  the  specimen  between 
the  fingers  of  the  left  hand  until  the  sting  protrudes,  then  grip 
it  with  a  pair  of  fine  forceps,  and  gently  pull  it  out.  If  properly 
done,  the  poison  gland  and  duct  should  come  away  with  it. 
Wash  in  water,  and  place  it  on  a  slide  under  a  dissecting  micro- 
scope, and  with  a  fine  needle-point  draw  the  stings  from  their 
sheath ;  this  is  done  by  putting  the  needle  under  the  stings  at 
the  base  of  the  sheath  and  carefully  drawing  it  towards  the  apex. 
Stain  in  borax  carmine,  wash  in  alcohol,  then  in  acidulated 
alcohol,  and  place  in  water ;  now  lay  out  on  a  slide,  place 
another  slide  over  it,  tie  with  thread,  and  immerse  in  methylated 
spirit  for  several  hours  ;  remove  from  glass,  clear  in  clove  oil, 
and  mount  in  Canada  balsam. 

Small  insects,  such  as  parasites,  may  be  mounted  whole  in  a 
cell  in  glycerine  without  treatment  with  potash,  so  that  their 
internal  organs  may  be  seen  in  situ,  but  they  usually  require 
clearing.  Take  of  Calvert's  carbolic  acid,  solid  at  ordinary 
temperatures,  2  ounces,  melt,  and  add  about  §  a  drachm  of 
glycerine  to  prevent  it  becoming  solid  again.  Soak  the  insect  in 
this  until  transparent ;  some  specimens  will  only  require  an  hour 
or  two,  others  a  week  or  more.  When  clear,  make  a  cell  as 
previously  directed  with  any  good  shellac  cement,  and  when  dry, 
run  on  a  coat  of  cement  to  its  upper  surface ;  let  this  become 
about  half  dry,  then  place  in  the"  cell,  fill  it  up  with  glycerine, 
and  apply  a  cover-glass,  which  must  be  carefully  pressed  down 
with  a  needle-point  until  it  adheres  to  the  cement  all  round. 
The  slide  can  then  be  washed  with  water  to  remove  all  trace  of 
excess  of  glycerine  ;  put  away  until  all  the  water  has  evaporated, 


202  MODEEN  MICROSCOPY 

then  apply  a  coat  of  shellac  cement,  and  when  this  has  dried, 
rub  away  any  water-marks  that  may  be  left  on  the  slide  with  a 
soft  cloth,  and  add  another  coat  of  cement. 

Wing-cases,  legs,  heads,  and  feet  of  diamond  beetles  should 
be  mounted  in  opaque  cells  in  Canada  balsam.  Take  a  slide, 
and  with  a  turn-table  run  on  a  disc  of  black  varnish  of  the 
required  size  ;  allow  this  to  dry  thoroughly.  Take  a  piece  of 
black  gummed  paper  and  punch  out  a  disc  of  the  same  size  as 
that  on  the  slide  to  which  it  is  to  be  fastened.  Now  take  a  tin 
cell  of  the  required  depth — on  no  account  use  brass  or  vulcanite 
cells ;  they  are  affected  by  the  balsam,  and  the  mount  will  be 
spoiled — lay  the  cell  on  a  slide,  and  apply  a  coat  of  cement  to 
its  upper  surface  ;  allow  this  to  become  nearly  dry,  then  take 
up  the  cell  in  a  pair  of  forceps,  and  bring  its  cemented  surface 
in  contact  with  the  paper  disc  on  the  slide,  and  with  the  point 
of  the  forceps  press  the  cell  down  until  the  cement  adheres  to 
the  paper.  Now  put  away  to  dry  in  some  place  protected  from 
dust.  Take  the  specimen  to  be  mounted,  examine  it  under  a 
microscope,  and  if  dirty  wash  in  some  benzole,  and  then  let  it 
dry  again.  Now  place  a  small  quantity  of  gum-water  in  the 
centre  of  the  cell,  and  put  the  specimen  into  it  in  the  desired 
position  ;  make  sure  that  it  adheres  securely  to  the  gum,  and  put 
the  slide  away  again  until  everything  is  quite  dry.  Put  the  slide 
in  a  turn-table,  and  run  on  a  coat  of  shellac  cement  to  its  upper 
surface,  and  allow  it  to  become  nearly  dry ;  then  fill  up  the  cell 
with  Canada  balsam,  clean,  and  apply  a  cover-glass,  which  must 
be  well  pressed  into  the  cement  until  it  adheres  firmly  ;  put  away 
for  an  hour,  and  then  wash  away  the  exuded  balsam  with  a  soft 
brush  and  some  turpentine  ;  dry  the  slide  with  a  soft  rag,  and 
apply  a  coat  of  black  shellac  cement. 

Heads  of  flies  having  coloured  compound  eyes,  such  as  Tabanus, 
lace-wing  flies,  etc.,  should  be  mounted  in  opaque  cells  in 
glycerine.  Make  the  cell  in  exactly  the  same  way  as  directed 
for  balsam  mounts,  but  take  care  that  the  cell  is  only  just  deep 
enough  to  take  the  specimen,  as  the  object  has  to  be  retained  in 
the  centre  of  the  cell  by  slight  pressure  on  the  part  of  the  cover- 
glass.  When  the  cell  is  quite  dry,  apply  a  coat  of  shellac  cement 
to  its  upper  surface,  and  let  it  nearly  dry  ;  then  take  a  brush 
and  some  clean  water  and  moisten  the  inside  of  the  cell.     This 


PKEPAKING  ENTOMOLOGICAL  SPECIMENS       203 

is  done  to  prevent  the  formation  of  air-bubbles,  for  if  glycerine 
is  put  into  a  dry  cell,  bubbles  are  sure  to  give  a  lot  of  trouble. 
Now  fill  the  cell  with  glycerine  and  put  in  the  specimen,  which 
should  be  previously  soaked  in  dilute  glycerine  for  an  hour  or 
two,  and  with  a  needle  place  it  in  the  desired  position  ;  apply  the 
cover-glass  very  carefully,  so  that  no  air-bubbles  may  be  enclosed, 
and  let  it  settle  down  by  its  own  weight  until  it  rests  on  the 
surface  of  the  cell ;  then  press  it  down  with  a  needle-point  until 
securely  embedded  in  the  half-dried  cement,  and  set  aside  for 
an  hour  or  two  to  dry.  The  exuded  glycerine  may  then  be  washed 
away  by  holding  the  slide  under  a  water-tap.  When  all  trace  of 
glycerine  is  removed,  dry  the  slide  with  a  soft  cloth,  and  apply  a 
coat  of  black  shellac  enamel. 

Heads  of  large  insects  may  be  secured  in  the  centre  of  the  cell 
in  the  following  way  :  Take  a  fine  needle,  thread  it  with  a  hair, 
and  run  it  through  the  specimen.  Unthread  the  needle,  take  up 
each  end  of  the  hair  with  the  object  suspended  and  stretch  it 
across  the  cell  so  that  it  may  be  embedded  in  the  cement  on  each 
side.  Now  apply  a  cover-glass,  press  it  down  until  securely  fixed, 
and  if  the  specimen  is  not  in  the  middle  of  the  cell,  adjust  it  by 
pulling  on  the  hair  on  one  side.  Put  away  to  dry,  cut  off  the 
ends  of  the  hair  close  to  the  edge  of  the  cell,  wash  away  excess  of 
glycerine,  dry,  and  apply  a  coat  of  shellac  enamel. 


CHAPTER  XVII 

CEYSTALS  AND  POLAKISCOPE  OBJECTS 

Crystals. — Method  1. — Make  a  strong  solution  of  the  material 
in  distilled  water,  with  the  aid  of  heat  if  necessary,  and  filter ; 
take  up  a  small  quantity  of  the  solution  in  a  dipping-tube,  and 
drop  it  on  a  cover-glass.  Prepare  several  covers  in  this  way, 
and  allow  some  to  dry  slowly,  and  evaporate  others  over  a  spirit- 
lamp.  When  dry,  add  a  drop  or  two  of  Canada  balsam,  and 
mount  in  the  usual  way. 

Method  2. — Make  a  strong  solution  in  distilled  water,  and  add 
a  few  drops  of  gum  water  or  a  small  piece  of  gelatine  ;  mix  well, 
and  filter.  Apply  some  of  the  solution  to  a  cover-glass,  and 
allow  it  to  dry  slowly  in  a  place  protected  from  dust.  Mount 
in  Canada  balsam. 

Method  3. — Place  a  small  piece  of  the  dry  crystal  on  a  slide, 
and  apply  a  cover-glass ;  warm  over  a  spirit-lamp  until  fusion 
results,  press  the  cover  down  with  a  needle,  and  allow  the  slide 
to  cool.  Clean  off  the  exuded  material,  and  finish  off  with  some 
good  cement. 

Some  crystals  are  soluble  in  Canada  balsam  ;  in  which  case, 
mount  in  castor  oil. 

Crystallize  the  specimen  on  the  cover-glass ;  make  a  thin  cell 

with  some  shellac  cement  on  a  slide,  and  allow  it  to  become 

perfectly  dry  ;   then  apply  another  coat  of   cement,  and  when 

this  has  nearly  dried,  fill  the  cell  with  castor  oil.     Take  up  the 

cover  with  a  pair  of  forceps,  and  bring  the  crystallized  surface 

in  contact  with  the  oil,  being  very  careful  that  no  air-bubbles 

form.     Ease  it  down  gently,  and  when  it  rests  on  the  cell,  give 

it  a  press  with  the  point  of  the  forceps ;  this  will  squeeze  out  the 

excess  of  oil  and  embed  the  edge  of  the  cover  in  the  cement. 

204 


CRYSTALS  AND  POLAEISCOPE  OBJECTS    205 

Put  away  to  dry  ;  wash  off  the  exuded  oil  with  some  turpentine, 
and  apply  another  coat  of  shellac  cement. 

The  following  salts,  etc.,  are  easily  obtained,  and  they  all  give 
very  good  results : 

Chloride  of  barium.*  Sulphate  of  iron.*  Asparagine. 

Chlorate  of  potash.*  Tartrate  of  soda.*  Quinidine. 

Sulphate  of  copper.*  Salicine.  Santonine. 

Spermaceti  (fuse).  Stearine  (fuse).  Tartaric  acid. 

Those  marked  *  are  more  effective  when  crystallized  in  gum 
or  gelatine. 

Crystals  of  Silver. — Clean  a  cover-glass  and  fasten  it  to  a 
slide  with  the  breath ;  make  a  1  per  cent,  solution  of  nitrate  of 
silver,  and  place  a  drop  of  it  in  the  centre  of  the  cover-glass. 
Now  add  a  very  small  fragment  of  copper,  and  put  the  slide 
away  out  of  reach  of  dust  until  the  crystals  have  formed  and  all 
moisture  has  evaporated.  Then  make  a  shallow  opaque  cell, 
and  place  a  small  drop  of  gum  water  in  its  centre.  Take  up  the 
cover  with  a  pair  of  forceps — crystals  uppermost,  of  course — and 
drop  it  into  the  cell;  now  take  a  needle-point,  and  carefully 
press  on  the  cover-glass  between  the  crystals,  until  it  lies  quite 
flat,  and  air-bubbles,  if  any,  have  exuded.  Put  the  slide  away 
again  until  the  gum  has  dried.  Now  put  the  slide  into  a  turn- 
table ;  run  on  a  coat  of  shellac  cement  to  the  upper  surface  of 
the  cell.  Allow  this  to  become  half  dry,  and  then  apply  a  cover- 
glass. 

The  following  specimens  from  the  vegetable  kingdom  make 
fine  polariscope  objects :  Starches,  hairs,  scales  from  leaves, 
cotton  and  silk  fibres,  cuticles  of  leaves,  and  longitudinal  and 
transverse  sections  of  stems. 

Starches  can  be  obtained  from  most  vegetable  substances  by 
scraping  the  cut  surface  with  a  knife.  Place  the  scrapings  in  a 
bottle  of  water  and  shake  well ;  then  strain  through  muslin  of 
sufficiently  fine  texture  to  allow  the  starch  to  pass,  but  to  retain 
the  fibres.  Now  put  the  strained  material  into  a  bottle,  shake  it 
up,  and  then  allow  to  settle ;  the  starch  will  fall  to  the  bottom  of 
the  bottle  in  a  few  minutes.  Then  pour  off  the  water ;  add  some 
more,  and  repeat  the  process  until  all  trace  of  cellular  tissue  is 


206  MODERN  MICROSCOPY 

removed.  When  the  starch  is  quite  clean,  take  up  a  little  in  a 
dipping-tube  ;  apply  a  drop  to  a  clean  cover.  See  that  it  spreads 
evenly  all  over  the  surface  of  the  cover,  and  put  away,  protected 
from  dust,  until  quite  dry ;  then  add  a  drop  of  Canada  balsam, 
and  mount  in  the  ordinary  way. 

Starches  may  also  be  mounted  in  glycerine  jelly  (see  p.  188), 
but  they  do  not  polarize  so  well  as  the  balsam  preparations. 

Sections  of  Starch- Bearing  Tissues. — The  stems,  roots,  and 
bulbs  must  be  hardened  in  methylated  spirit  for  a  week  ;  then 
make  transverse  or  longitudinal  sections.  Dehydrate  in  methy- 
lated spirit,  clear  in  clove  oil,  and  mount  in  Canada  balsam. 

Cuticles  containing  Raphides. — The  most  common  are  taken 
from  the  following  bulbs  :  garlic,  onion,  lily,  hyacinth.  Strip 
off  the  cuticle  from  the  fresh  specimen  ;  dehydrate  in  methy- 
lated spirit,  clear  in  clove  oil,  and  mount  in  Canada  balsam. 

Cuticles  of  Leaves. — Cut  up  the  leaf  into  small  pieces,  and 
soak  in  water  until  rotten ;  the  cuticles  can  then  be  separated, 
washed  in  water,  dehydrated  in  methylated  spirit,  cleared  in 
clove  oil,  and  mounted  in  Canada  balsam. 

Cotton,  Hemp,  Wool,  Silk,  Flax,  etc. — Place  the  fibres  in 
methylated  spirit  to  dehydrate ;  then  clear  in  clove  oil,  and  place 
a  little  on  a  slide.  Separate  the  fibres  from  each  other  with 
needle-points ;  apply  a  few  drops  of  Canada  balsam  and  a  cover- 
glass. 

Scales  of  Leaves. — Scrape  the  leaf  with  a  knife,  and  put  the 
scrapings  into  a  bottle  of  turpentine,  and  soak  until  all  trace  of 
air  has  disappeared  from  the  scales  ;  then  pour  off  the  turpentine. 
Take  up  a  little  of  the  scales  on  the  point  of  a  penknife,  and 
mount  them  in  Canada  balsam  in  the  ordinary  way.  Some 
leaf-scales  are  very  difficult  to  deprive  of  air ;  in  fact,  it  is 
impossible  to  get  them  quite  free. 

The  following  animal  tissues  make  good  polariscope  objects  : 
fish-scales,  palates  of  molluscae,  sections  of  hairs  and  quills,  horns 
and  hoofs,  whalebone,  claws  of  dogs,  cats,  and  fowls,  decalcified 
bones,  muscular  tissues. 

Fish  Scales. — Scrape  the  fish  from  the  head  towards  the  tail ; 
if  scraped  the  other  way,  nearly  all  the  scales  will  be  injured. 
Place  the  scrapings  in  a  bottle  of  water,  shake  well,  pour  off  the 
water,  and  repeat  the  process  until  quite  clean.     Examine  with  a 


CEYSTALS  AND  POLAEISCOPE  OBJECTS    207 

microscope,  and  if  you  find  that  the  scales  are  not  clean,  pour 
off  the  water,  add  liq.  potassae,  and  soak  for  an  hour  or  two  ;  then 
wash  away  the  potash  with  repeated  changes  of  water,  dehydrate 
in  methylated  spirit,  clear  in  clove  oil,  and  mount  in  Canada 
balsam. 

Sometimes  fish  scales  buckle  up  in  spirit,  and  they  will  not  lie 
flat.  When  this  happens,  put  them  into  water  again,  and  soak 
a  little  while  ;  then  place  them  on  a  slide,  and  put  another  slide 
over  them,  press  down  until  quite  flat,  and  tie  the  two  glasses 
together  with  twine,  and  place  them  in  a  vessel  of  methylated 
spirit  to  dehydrate  under  pressure.  This  method  will  answer 
for  all  tissues  that  have  a  tendency  to  twist  during  the  process 
of  dehydration. 

Palates. — Dissect  out,  and  soak  in  liq.  potassse  for  a  few  days. 
Wash  well  in  water,  spread  out  on  a  slide ;  put  a  piece  of  paper 
on  each  side  of  it  to  prevent  crushing,  and  place  another  slide 
over  all  in  the  same  way  as  directed  for  insects  ;  tie  the  glasses 
together  with  string,  and  place  in  methylated  spirit  for  an  hour 
or  two.  Then  remove  the  palate  from  the  glasses,  and  place  it 
in  clove  oil  until  clear.     Mount  in  Canada  balsam. 

Sometimes  it  is  very  difficult  to  dissect  out  the  palates  from 
small  snails.  This  process  answers  just  as  well :  Cut  off  the 
head  of  the  animal,  being  careful  that  you  remove  the  buccal 
mass  with  it,  and  place  in  liq.  potassse  for  a  few  days  ;  this  will 
destroy  all  the  soft  tissues,  but  not  the  palate  or  radula.  Wash 
away  the  potash  with  repeated  changes  of  water,  and  proceed  as 
directed  above. 

Sections  of  Hairs  and  Quills  may  sometimes  be  cut  after 
soaking  for  a  few  days  in  methylated  spirit ;  but  some  of  the 
larger  kinds,  such  as  the  whisker  of  walrus,  will  require  softening 
in  potash.  Place  in  liq.  potassse  for  a  few  hours  or  days,  in 
accordance  with  the  consistency  of  the  tissue.  When  soft  enough, 
wash  away  the  potash  with  water,  and  place  in  methylated  spirit, 
in  which  they  may  be  preserved  until  required.  Then  make 
transverse  and  longitudinal  sections,  dehydrate  in  methylated 
spirit,  clear  in  clove  oil,  and  mount  in  Canada  balsam. 

Small  Fine  Hairs. — Cut  off  a  number  of  hairs,  tie  them  up 
into  a  bundle  with  some  cotton,  and  soak  for  a  few  minutes  in 
warm  water.     Make  up  a  strong  solution  of  gelatine  in  water,  and 


208  MODEEN  MICEOSCOPY 

transfer  the  bundle  of  hairs  to  it,  and  soak  it  for  several  hours 
in  a  hot- water  bath  until  the  gelatine  has  penetrated  to  the  centre 
of  the  bundle.  Eeruove  from  the  gelatine  on  the  point  of  a 
needle,  and  hold  it  exposed  to  the  air  until  the  gelatine  has 
cooled ;  then  push  them  from  off  the  needle  into  a  bottle  of 
methylated  spirit,  and  soak  for  an  hour  or  two  to  complete  the 
hardening.  Embed  in  carrot,  put  in  a  microtome,  and  cut 
transverse  sections,  and  as  they  are  cut  place  them  in  methylated 
spirit  to  dehydrate  ;  then  clear  in  clove  oil,  and  mount  in  Canada 
balsam. 

Horns,  Hoofs,  Whalebone,  and  Claws  all  require  steeping 
in  liq.  potassae  until  soft ;  they  are  then  to  be  washed  in  water, 
and  preserved  in  methylated  spirit  until  required.  Embed  in 
carrot,  place  in  a  well  microtome,  make  transverse  and  longi- 
tudinal sections,  dehydrate  in  methylated  spirit,  clear  in  clove 
oil,  and  mount  in  Canada  balsam. 

Decalcified  Bones  (see  Chapter  VII.). — Embed  in  carrot, 
make  transverse  and  longitudinal  sections,  dehydrate  in  methy- 
lated spirit,  clear  in  clove  oil,  and  mount  in  Canada  balsam. 

Muscular  Fibres. — Take  the  tongue  of  a  cat,  harden  it  in 
methylated  spirit  for  a  week  or  ten  days  ;  then  embed  in  carrot, 
and  make  transverse  or  longitudinal  sections,  dehydrate,  clear  in 
clove  oil,  and  mount  in  Canada  balsam. 


CHAPTER  XVIII 

CLEANING  AND   MOUNTING   DIATOMS,  POLYCYSTINA, 

AND  FOKAMINIFEKA 

To  Clean  Diatoms  growing  upon  Algae  or  Shells. — Place 
the  algre  or  shells  in  a  basin,  cover  them  with  water,  add  hydro- 
chloric acid,  and  stir  until  effervescence  results ;  add  more  acid 
little  by  little,  until  effervescence  ceases,  stirring  from  time  to 
time.  Now  strain  through  net  of  sufficiently  fine  texture  to 
allow  the  diatoms  to  pass,  but  to  retain  the  debris.  Allow  the 
strained  fluid  to  settle  down,  pour  off  the  acid  water,  and  place 
the  deposit  in  a  large  test-tube.  Add  pure  hydrochloric  acid, 
and  boil  for  twenty  minutes ;  add  some  pure  nitric  acid,  and 
boil  again  for  twenty  minutes,  and,  while  boiling,  add  some 
crystals  of  chlorate  of  potash  until  complete  bleaching  results. 
Eemove  all  trace  of  acid  or  alkali  by  washing  in  water,  and 
examine  the  forms  under  the  microscope.  If  clean,  bottle  them 
up  in  distilled  water  for  future  mounting.  If,  as  is  sometimes 
the  case,  there  has  been  animal  matter  present  which  has  not 
been  removed,  boil  in  pure  sulphuric  acid  for  a  few  minutes. 
Wash  away  all  trace  of  acid  before  bottling  the  diatoms  in 
distilled  water. 

To  Clean  Fossil  Diatomaceous  Deposits. — Break  the 
deposit  up  into  small  pieces,  and  place  them  in  a  large  test- 
tube  in  a  moderately  strong  solution  of  bicarbonate  of  soda,  and 
boil  gently  for  two  hours,  the  disintegrated  portions  being  from 
time  to  time  poured  off  into  a  beaker  and  the  boiling  in  soda 
continued  until  all  the  deposit  has  broken  up.  The  alkaline 
solution  must  then  be  washed  way,  and  the  diatoms  boiled  for 
a  short  time  in  nitric  acid,  and  when  sufficiently  clean  wash 

209  14 


210  MODERN  MICEOSCOPY 

away  the  acid  in  repeated  changes  of  water,  and  bottle  up  the 
diatoms  in  distilled  water. 

To  Clean  Living  Diatoms. — Remove  all  dirt  or  salt  by  wash- 
ing well  in  water;  shake  well,  and  allow  the  diatoms  to  settle 
before  pouring  off  the  water.  In  this  way  all  soluble  impurities 
can  be  removed.  When  the  water  remains  clear,  pour  it  off, 
leaving  the  diatoms  as  nearly  dry  as  possible,  and  cover  them 
with  strong  alcohol,  which  will  extract  the  endochrome  ;  change 
the  alcohol  daily  until  it  ceases  to  be  tinged  with  green  ;  then 
wash  away  the  alcohol  with  water,  pour  off  the  water,  and  place 
the  diatoms  in  a  platinum  capsule  and  heat  them  to  a  dull  red 
over  a  spirit-lamp.  This  will  separate  the  frustules  into  single 
valves,  and  finish  the  cleaning  of  the  diatoms,  and  they  may 
then  be  bottled  up  in  distilled  water. 

To  Clean  Polycystina. — The  polycystinous  earth  should  be 
broken  into  small  pieces  and  boiled  for  several  hours  in  a 
strong  solution  of  common  washing  soda,  the  disintegrated 
matter  being  from  time  to  time  poured  off  into  a  vessel,  and 
the  boiling  in  soda  continued  until  all  the  earth  is  broken  up. 
Wash  the  disintegrated  matter  in  water  several  times  to  remove 
the  soda,  allow  the  polycystina  to  settle  down,  and  pour  off  the 
water  and  place  the  forms  in  a  test-tube ;  add  some  nitric  acid, 
and  boil  for  twenty  minutes.  Remove  all  trace  of  acid  with 
water,  and  bottle  up  in  distilled  water. 

To  Clean  Foraminifera. — All  mud  must  be  got  rid  of  by 
repeated  washing  in  water.  Then  boil  the  forms  in  a  strong 
solution  of  bicarbonate  of  soda  for  an  hour  or  two.  When  clean, 
wash  away  the  soda,  and  bottle  in  distilled  water. 

To  Mount  Diatoms  in  Canada  Balsam  (Unselected  Slides). 
— The  diatoms  are  to  be  taken  out  of  the  bottle  with  a  dipping- 
tube,  and  should  be  allowed  to  fall  upon  a  clean  cover-glass. 
The  fall  of  the  drop  causes  the  forms  to  spread  evenly  over  the 
cover.  It  should  then  be  dried  slowly  over  a  spirit-lamp.  When 
dry  a  small  drop  of  Canada  balsam  is  to  be  applied,  and  the 
slide  put  away  out  of  reach  of  dust  to  dry  for  twelve  hours.  Now 
place  on  a  hot  plate,  and  apply  gentle  heat  from  a  spirit-lamp 
for  about  ten  minutes.  Allow  it  to  cool.  Take  the  cover  up 
with  a  pair  of  forceps,  and  bring  its  balsamed  surface  in  contact 
with  the  centre  of  a  warmed  slide.     The  balsam  should  then  run 


CLEANING  AND  MOUNTING  DIATOMS,  ETC.      211 

to  a  neat  bevelled  edge  all  round  the  cover  ;  should  it  not  do  so, 
warm  the  slide  a  little  more  until  it  does. 

Unselected  Polycystina. — Take  the  forms  from  the  bottle 
with  a  glass  tube,  and  spread  them  on  a  slide ;  dry  them  over  a 
spirit-lamp.  Now  clean  a  cover-glass,  fasten  it  to  a  slip  with 
your  breath,  and  place  a  drop  or  two  of  balsam  on  it ;  take  up 
some  of  the  polycystina  on  the  point  of  a  knife  and  place  them 
in  the  balsam ;  stir  them  well  up  with  a  needle  and  put  away 
for  twelve  hours.  Bake  over  a  spirit-lamp  for  ten  minutes,  and 
while  warm  stir  up  again  gently  with  a  needle,  and  spread  the 
forms  evenly  over  the  cover.  Warm  a  glass  slide,  and  proceed 
as  directed  for  unselected  diatoms. 

Unselected  Polycystina  as  Opaque  Objects. — Dry  some 
polycystina  on  a  slide,  then  take  a  platinum  capsule,  put  the 
dried  material  into  it,  and  heat  over  a  spirit-lamp  to  a  dull  red. 
Clean  a  cover-glass,  fasten  it  to  a  slide  with  your  breath,  and 
apply  a  few  drops  of  balsam.  Take  up  some  of  the  dried  forms, 
put  them  into  the  balsam,  and  stir  up  with  a  needle  until  they 
are  evenly  spread  over  the  cover ;  put  away  out  of  reach  of  dust 
for  twelve  hours,  so  that  the  air  may  escape  from  the  forms. 
Now  place  on  a  hot  plate  and  apply  gentle  heat  for  ten  to  fifteen 
minutes  to  bake  the  balsam.  Clean  another  cover-glass,  add  a 
drop  or  two  of  balsam  to  the  hardened  balsam,  and  apply  the 
second  cover-glass ;  warm  again,  and  with  a  needle  press  gently 
on  the  upper  cover  until  it  lies  perfectly  flat ;  then  allow  to  cool, 
apply  a  coat  of  black  shellac  cement  all  over  one  side  of  the 
upper  cover,  and  put  away  to  dry.  In  the  meantime  take  a  slip, 
put  it  in  a  turn-table,  and  run  on  a  disc  of  black  varnish  of  the 
same  size  as  the  cover ;  let  this  dry,  then  add  a  drop  of  strong 
gum  or  glue ;  take  up  the  covers  with  a  pair  of  forceps,  and  put 
the  blackened  side  into  the  glue  ;  press  down  with  a  needle  until 
the  glue  spreads  evenly  under  the  cover,  and  put  away  to  dry. 
When  dry,  finish  off  with  a  coat  of  black  cement. 

Selected  Diatoms  and  Polycystina. — Take  an  ounce  of  dis- 
tilled water,  add  6  or  8  drops  of  ordinary  gum  water,  and  filter. 
Clean  a  cover-glass,  and  place  a  drop  of  the  diluted  gum  upon  it ; 
put  away  to  dry. 

Spread  the  diatoms  or  polycystina  on  a  slip,  and  dry  them 
over  a  spirit-lamp.     Select  the  desired  forms  with  a  line  brush 


21-2  MODERN  MICROSCOPY 

or  bristle,  and  breathe  upon  the  gummed  surface  of  the  cover, 
and  place  the  forms  upon  it.  When  dry,  apply  a  drop  of  balsam, 
and  put  away  out  of  reach  of  dust  for  twelve  hours.  Bake  and 
finish  as  directed  for  unselected  slides. 

In  mounting  selected  polycystina,  they  must  be  between  two 
covers  ;  if  on  a  single  cover,  the  forms  would  be  upside  down 
when  the  cover  was  reversed.  If  a  transparent  mount  is  desired, 
the  two  covers  can  be  fastened  to  the  slide  with  a  drop  of  balsam. 
If  opaque,  the  forms  must  be  burnt,  and  one  side  must  be 
blackened ;  in  other  respects  proceed  exactly  as  you  would  for 
unselected  opaque  mounts. 

Polycystina  may  also  be  mounted  in  a  dry  opaque  cell.  Take 
a  slide,  run  on  a  disc  of  black  varnish,  and  when  this  has  dried 
fasten  a  disc  of  black  gummed  paper  over  it.  Then  take  a 
shallow  cell,  apply  a  coat  of  cement  to  one  side  of  it,  and  let  it 
nearly  dry  ;  then  fasten  to  the  paper  disc,  and  put  away  to  dry. 
Apply  a  little  dilute  gum  water  to  the  bottom  of  the  cell,  select 
the  specimens,  and  put  them  into  the  gum  ;  if  they  do  not 
adhere,  breathe  on  the  surface  of  the  gum.  When  all  are  arranged, 
put  the  slide  away  until  everything  is  quite  dry  ;  then  add  a  coat 
of  cement  to  the  upper  side  of  the  cell,  let  it  nearly  dry,  and 
then  apply  the  cover-glass. 

Foraminifera — Unselected  Transparent  Mounts. — Dry  the 
forms  on  a  slide  with  the  aid  of  gentle  heat,  and  scrape  them  off 
into  a  bottle  of  turpentine,  in  which  they  must  soak  until  all 
trace  of  air  has  disappeared.  Then  clean  a  cover-glass,  fasten 
it  to  a  slide  with  condensed  breath,  and  apply  a  few  drops  of 
balsam.  Pour  off  the  turpentine  from  the  foraminifera,  take  up 
some  of  the  forms  on  the  point  of  a  penknife,  and  put  them  into 
the  balsam  on  the  cover  ;  stir  up  with  a  needle  until  spread  evenly, 
then  put  away  for  twelve  hours.  Bake  gently  for  ten  minutes 
on  a  hot  plate,  cool,  apply  a  drop  of  fluid  balsam,  warm  a  slide 
over  a  spirit-lamp,  take  the  cover  up  in  a  pair  of  forceps,  and 
bring  the  drop  of  fluid  balsam  in  contact  with  the  centre  of  the 
slide,  ease  down  carefully,  and  press  on  the  upper  surface  of  the 
cover  with  the  needle-point  until  it  lies  quite  flat ;  or  if  the  forms 
are  very  delicate,  warm  the  slide  again  gently  until  the  cover 
settles  down  by  its  own  weight.     Allow  the  slide  to  cool,  then 


CLEANING  AND  MOUNTING  DIATOMS,  ETC.      213 

clean  away  exuded  balsam  with  methylated  spirit,  and  apply  a 
coat  of  cement. 

Foraminifera — Opaque  Mounts. — Proceed  in  exactly  the  same 
way  as  directed  for  mounting  dry  opaque  polycystina  ;  but  if  the 
specimens  are  unselected,  gum  the  bottom  of  the  cell,  dry  the  forms 
on  a  slide,  and  spread  a  quantity  of  them  all  over  the  surface  of 
the  cell.  Let  the  gum  dry,  then  shake  out  all  that  have  not 
adhered,  apply  a  coat  of  cement  to  the  upper  side  of  the  cell, 
and  when  this  has  nearly  dried,  apply  a  cover-glass. 

Spicules  of  Gorgonia  or  Sea-Fan. — Boil  in  liq.  potassae 
until  all  the  material  has  broken  up,  then  wash  away  the  potash 
with  repeated  changes  of  water,  allowing  the  spicules  to  settle  to 
the  bottom  of  the  tube  between  each  washing.  When  cleaned, 
preserve  in  a  bottle  of  dilute  spirit.  Proceed  with  the  mounting 
in  exactly  the  same  way  as  directed  for  transparent  unselected 
polycystina. 

Spicules  of  Alcionium. — Proceed  as  above. 

Sponges. 

1.  To  Show  Cell  Structure,  Flagellated  Cell,  etc. — Fresh 
specimens  of  the  calcareous  forms — Sycon,  for  example — should 
be  fixed  with  osmic  acid  1  per  cent,  solution,  washed  in  distilled 
water,  and  placed  in  absolute  alcohol  for  twelve  hours  ;  then 
soak  in  absolute  alcohol  and  ether  for  a  few  hours,  infiltrate, 
and  embed  in  celloidin.  Cut  sections  in  a  microtome.  Place 
sections  in  absolute  alcohol  for  about  three  minutes,  clear  in  oil 
of  origanum,  and  mount  in  Canada  balsam. 

If  preferred,  sections  of  sponges  may  be  mounted  in  glycerine 
jelly,  but  they  must  be  soaked  in  water  for  a  little  while  before 
they  can  go  into  the  jelly. 

2.  The  Skeleton — (a)  Horny  Sponges. — Boil  in  liq.  potassae, 
then  wash  the  spicules  well  in  water,  and  mount  in  glycerine 
jelly  or  Canada  balsam. 

(b)  Calcareous. — Dehydrate  small  forms  in  alcohol,  clear  in 
clove  oil,  and  mount  in  Canada  balsam  in  a  cell ;  or  separate 
the  spicules  by  boiling  in  liq.  potassa3,  wash  in  water,  and 
mount  in  Canada  balsam  or  glycerine  jelly. 


214  MODERN  MICROSCOPY 

(c)  Siliceous. — Boil  in  nitric  acid,  wash  well  in  water,  dehydrate, 
clear,  and  mount  in  Canada  balsam. 

For  the  types  in  which  siliceous  spicules  are  embedded  in 
horny  material,  boil  in  liq.  potassae  for  a  few  minutes  to  disin- 
tegrate the  tissues,  then  in  nitric  acid  to  clean  the  spicules, 
wash  well  in  water,  and  mount  in  Canada  balsam. 

Sections  of  Sponges. — Harden  in  methylated  spirit,  and 
transfer  to  equal  parts  of  ether  and  absolute  alcohol  for  several 
hours.  Then  place  in  a  thin  solution  of  celloidin  for  a  day  or 
two,  transfer  to  a  thicker  solution  of  celloidin,  and  soak  again 
for  a  few  hours.  Remove  from  the  celloidin  on  the  point  of  a 
needle,  and  hold  exposed  to  the  air  for  a  few  minutes  to  allow 
the  celloidin  to  set  around  the  specimen ;  then  push  it  off  the 
needle  into  a  bottle  of  methylated  spirit,  and  soak  for  a  few 
hours  to  complete  the  hardening.  Embed  in  carrot,  place  in  a 
well  microtome,  and  make  the  sections.  Dehydrate  in  methy- 
lated spirit,  clear  in  oil  of  bergamot,  and  mount  in  Canada 
balsam. 

Sometimes  sponge  sections  are  rendered  too  transparent  by 
mounting  in  balsam.  In  such  cases,  mount  in  glycerine  jelly, 
but  be  careful  to  wash  away  all  traces  of  alcohol  before  they  go 
into  the  jelly. 


CHAPTER  XIX 

DEY    MOUNTS 

Opaque  Cells. — Place  a  slide  in  a  turn-table,  and  run  a  disc 
of  black  varnish  on  its  centre ;  allow  this  to  dry.  Take  a  piece 
of  black  paper  and  punch  out  a  disc  of  the  same  size  as  the  one 
on  the  slide,  and  gum  it  on  to  the  varnish  spot.  Take  a  cell, 
either  metal  or  vulcanite,  of  the  required  depth  and  fasten  it  to 
the  paper  disc  with  gold  size  or  black  shellac  cement,  and  put 
the  slide  away  until  quite  dry.  Now  place  a  very  small  quantity 
of  gum  on  the  centre  of  the  paper  disc,  and  put  the  specimen 
into  it ;  but  take  care  that  the  gum  does  not  extend  beyond  the 
object,  or  the  appearance  of  the  mount  will  be  spoiled.  When 
the  gum  has  dried,  put  the  slide  into  the  turn-table  again,  and 
run  a  ring  of  any  good  cement  on  the  upper  surface  of  the  cell, 
and  when  this  has  become  about  half  dry  apply  a  cover-glass, 
which  must  be  pressed  down  with  a  needle-point  until  it  adheres 
firmly  to  the  cement  all  round  the  cell.  Put  the  slide  aside  for 
an  hour  or  two,  and  then  run  on  a  good  coat  of  black  shellac 
cement. 

Feathers  of  humming-birds,  eggs  of  butterflies  and  moths, 
small  microscopic  seeds,  gills  of  many  fishes,  skins  of  fishes, 
skins  of  snakes,  and  transverse  or  longitudinal  sections  of  stems 
of  plants,  are  all  mounted  as  opaque  objects  in  the  same  manner 
as  above.  The  former  should  be  arranged  in  the  cell  in  a  group. 
The  gills,  skins,  etc.,  should  be  well  washed  with  distilled  water 
and  dried  under  pressure  between  two  glass  slips  tied  together 
with  twine. 

Transparent  Cells. — Take  a  cell  of  the  desired  depth  and 
apply  a  coat  of  cement  to  one  side  of  it,  and  allow  it  to  become 
very  nearly  dry.     Take  a  slide  and  warm  it  gently  over  a  spirit- 

215 


216  MODEKN  MICEOSCOPY 

lamp ;  take  up  the  cell  with  a  pair  of  forceps  and  place  it  on  the 
centre  of  the  slide,  the  warmth  of  which  should  cause  the  cement 
of  the  cell  to  melt ;  if  not,  warm  a  little  more,  and  press  the  cell 
down  gently  with  a  needle-point  until  it  adheres  firmly  to  the 
slide  all  round.  If  the  specimen  is  small  it  must  be  fastened  in 
the  cell  with  some  gum,  as  for  opaque  mounts,  then  put  it  away 
until  the  gum  has  dried,  apply  a  cover,  and  finish  off  as  directed 
for  opaque  mounts.  Leaves  of  plants  and  wings  of  butterflies 
should  be  mounted  on  a  thin  slide,  so  that  both  sides  may  be 
examined.  No  gum  will  be  required  for  these  specimens,  but  a 
piece  of  the  leaf  or  wing  should  be  cut  or  punched  out  as  nearly 
the  size  of  the  cell  as  possible,  and  a  thin  cell  should  be  used, 
so  that  the  cover  may  rest  on  the  object  and  keep  it  flat.  In  all 
dry  mounts  great  care  must  be  taken  that  all  the  cements  used 
to  fasten  the  objects  in  position  are  quite  dry  before  the  cover 
is  put  on  ;  if  not,  any  moisture  remaining  will  condense  on  the 
under  surface  of  the  cover  and  spoil  the  preparation. 

Opaque  Mounts  of  Pollens. — Make  an  opaque  cell,  and 
apply  a  thin  layer  of  gum  water  all  over  its  floor  ;  then  take 
some  perfectly  dry  pollen  and  put  it  in  the  cell,  shake  the  slide 
so  that  the  pollen  spreads  evenly  all  over  the  cell,  and  let  it  dry. 
Then  apply  some  enamel  to  the  upper  surface  of  the  ring  of 
the  cell,  and  when  this  is  about  half  dry  apply  the  cover-glass. 


CHAPTEE  XX 

FINISHING  OFF  SLIDES 

Canada  Balsam — Quick  Method. — Take  a  small  saucer  of  chloro- 
form and  a  soft  brush,  and  carefully  wash  away  the  exuded 
balsam.  Allow  the  slide  to  dry,  then  place  it  in  a  turn-table 
and  apply  a  coat  of  black  shellac  cement.  Let  this  dry,  then 
wash  the  slide  quite  clean  with  turpentine  and  apply  another 
coat  of  cement. 

Canada  Balsam — Exposure  Method. — Put  the  slide  into  a 
saucer  of  methylated  spirit,  and  with  a  small  piece  of  soft  rag 
gently  rub  away  the  excess  of  balsam  ;  dry  the  slide  with  a 
clean  cloth,  and  apply  a  coat  of  any  good  cement. 

Glycerine  Jelly. — Put  the  slide  in  a  saucer  of  cold  water  and 
allow  it  to  soak  for  a  few  minutes,  then  take  a  penknife  and 
carefully  scrape  away  the  jelly  from  the  edge  of  the  cover.  Give 
the  slide  a  good  wash  in  water,  and  place  it  in  some  methylated 
spirit,  which  will  remove  the  water.  Dry  with  a  clean  soft  cloth, 
and  apply  a  coat  of  black  shellac  enamel,  and  when  this  has  dried 
add  another. 

Farrant's  Medium. — Allow  the  slide  to  dry  for  a  few  days, 
then  put  it  into  a  saucer  of  water  and  wash  away  the  excess  of 
medium  with  a  soft  brush.  Drain  off  as  much  water  as  possible, 
and,  if  the  cover  is  firm  enough,  dry  the  slide  carefully  with  a 
soft  cloth  ;  if  not,  allow  all  the  moisture  to  evaporate  by  exposure 
to  the  air.  When  quite  dry,  put  it  in  a  turn-table  and  apply  a 
coat  of  cement,  and  when  this  has  dried  add  another. 

Dry  Mounts  do  not  require  any  washing,  but  they  should 
have  one  or  two  coats  of  any  good  cement. 

Asphalte  and  white  zinc  cement  may  be  used  when  desired  for 

217 


218  MODERN  MICROSCOPY 

balsam  or  dry  mounts,  but  they  are  both  useless  for  any  of  the 
aqueous  or  fluid  media. 

A  really  good  black  enamel  may  be  made  in  the  following 
way : 

Dissolve  best  black  sealing-wax  in  methylated  spirit  until  the 
solution  is  as  thick  as  treacle,  then  mix  this  with  an  equal 
quantity  of  marine  glue  ;  then  if  too  thick,  dilute  with  a  little 
methylated  spirit.  This  cement  has  been  alluded  to  in  these 
chapters  as  shellac  cement,  and  it  is  the  best  I  know  of  for 
general  purposes.  The  black  enamel  should  be  kept  in  a  wide- 
mouthed,  stoppered  bottle.  Should  the  stopper  become  fixed, 
just  warm  the  neck  of  the  bottle  over  a  spirit-lamp ;  it  can  then 
be  easily  removed. 

When  a  ring  is  being  applied  to  a  slide,  the  turn-table  should 
not  be  run  too  fast,  and  the  extreme  point  of  the  brush  should 
only  just  touch  the  glass.  A  thin  coat  must  be  run  on  at  first, 
then  give  it  about  ten  minutes  to  dry.  A  sufficient  quantity 
of  cement  may  then  be  added  to  finish  the  mount,  but  if  too 
much  is  applied  at  first  it  will  overflow. 

The  most  suitable  brush  for  ringing  slides  is  a  sable  '  rigger ' 
No.  2  in  a  metal  holder  ;  it  should  be  well  washed  in  methylated 
spirit  after  use. 

Cleaning  off  Failures. — During  a  course  of  microscopical 
work  many  slides  will  be  not  worth  keeping,  but  the  slips  and 
covers  are  quite  good,  and  they  can  be  used  again.  When  a  batch 
of  failures  has  accumulated,  make  a  strong  solution  of  Hudson's 
soap-powder  in  warm  water,  and  place  some  of  it  in  two  jars. 
Warm  the  slide  over  a  spirit-lamp,  and  with  a  needle-point  push 
off  the  cover  into  one  of  the  jars  and  put  the  slip  into  the  other ; 
let  them  soak  for  an  hour  or  two,  then  wash  away  the  soap 
solution  with  repeated  changes  of  warm  water,  and  finally  pour 
away  all  the  water  and  add  methylated  spirit ;  soak  for  a  little 
while,  and  then  dry  with  a  soft  clean  rag. 

Sometimes  slips  and  covers  have  a  dull,  cloudy  appearance, 
which  defies  all  attempts  to  remove  it.  When  this  is  the  case, 
make  up  a  solution  of  hydrochloric  acid  in  methylated  spirit 
(about  one  part  of  acid  in  six  of  spirit),  and  immerse  the  glasses 
for  a  few  minutes.  Wash  away  the  acid  with  methylated  spirit, 
and  dry  with  a  soft  rag. 


PART    III 


CHAPTER     XXI 

AN  INTRODUCTION  TO  THE  USE  OF  THE  PETRO- 

LOGICAL  MICROSCOPE 

By  FREDERIC  J.  CHESHIRE,  F.R.M.S. 

The  petrological  microscope  is  nothing  more  than  an  ordinary 
microscope  fitted  with  certain  optical  and  mechanical  adjuncts, 
by  the  proper  employment  of  which  a  more  or  less  complete 
quantitative  and  qualitative  determination  can  be  made  of  the 
optical  properties  of  transparent  crystals,  as  exhibited  in  polar- 
ized light,  and  thus  the  identification  of  such  crystals  either 
effected  or  facilitated.  An  elementary  knowledge  at  least  of  the 
explanation  of  polarization  phenomena  is,  therefore,  imperative 
for  the  intelligent  and  efficient  use  of  the  microscope  in  question 
— a  fact  which  is  too  often  overlooked  by  the  microscopist. 

Polarization  of  Light. — It  is  a  very  curious  fact  that  the 
understanding  of  polarization  phenomena,  when  associated  with 
light-waves,  should  present  so  many  difficulties,  when  it  is 
remembered  that  polarized  waves  are  the  only  forms  of  wave- 
motion  with  which  most  people  are  made  familiar  by  their 
everyday  experiences.  A  pebble  dropped  into  a  pond  produces 
a  number  of  small  waves,  which,  starting  at  the  point  where 
the  pebble  fell  into  the  water,  spread  outwards  in  all  directions 
in  the  form  of  ever-increasing  circles,  until  they  have  passed 
over  the  entire  surface  of  the  pond.  But,  although  these  waves 
travel  over  the  surface,  we  know  that  the  particles  of  water  con- 
cerned in  their  production  do  not  travel,  but  simply  move  up  and 
down.   A  floating  cork,  for  example,  is  not  carried  forward  by  these 

219 


220 


MODERN  MICROSCOPY 


waves,  but  simply  rises  and  falls  as  each  wave  passes  beneath  it. 
Here,  then,  we  have  a  case  in  which  waves  are  propagated  from 
one  place  to  another  by  the  vibratory  motion  of  a  large  number 
of  particles  in  one  plane  only.  When  waves  are  transmitted  in 
this  way  by  the  motions  of  particles  in  a  single  and  constant 
plane,  the  wave  is  said  to  be  polarized.  But  a  simpler  and  even 
more  instructive  example  might  have  been  given. 

Let  us  suppose  that  a  long  rope  is  stretched  rather  loosely 
between  two  boys,  one  of  whom,  by  a  rhythmic  movement  of  his 
hand  in  an  up-and-down  direction,  produces  waves  in  the  rope 
which  pass  continuously  to  the  other  boy.     In  this  case,  too, 


Fig.  66. — Rope  Polariscope  :  Nicols  Parallel. 

since  the  vibrations  of  the  particles  of  which  the  rope  is  made 
up  take  place  in  one  constant  plane  only,  the  resulting  waves 
are  polarized  ones.  But  now,  as  in  Fig.  66,  let  the  rope  be 
passed  through  two  gratings,  A  and  B — such,  for  example,  as 
those  used  for  covering  street  gullies — in  both  of  which  the  bars 
are  at  first  arranged  vertically.  Now  let  the  first  boy  start 
waves  along  the  rope — not  by  confining  the  motion  of  his  hand 
to  one  plane  only,  but  by  changing  the  direction  both  rapidly 
and  arbitrarily,  so  that  a  jumble  of  waves  is  sent  along  the  rope 
in  which  the  vibrations  of  the  particles  take  place  more  or  less 
in  all  directions.  It  is  clear  that  these  waves  are  no  longer  polar- 
ized, since  the  various  particles  concerned  are  moving  in  different 
planes.    This  state  of  affairs  would  only  exist,  however,  between  the 


THE  USE  OF  THE  PETROLOGICAL  MICROSCOPE     221 


first  boy  and  the  grating  A,  for  since  the  bars  of  the  latter  are 
vertical,  it  is  clear  that  each  wave  as  it  falls  upon  the  grating  would, 
in  general,  be  partly  stopped  and  partly  transmitted ;  and,  since 
in  the  motion  transmitted  the  vibrations  would  take  place  in  one 
constant  plane  only,  it  would  be  polarized.  The  grating  A  would 
thus  act  as  a  polarizer,  and  we  should  have  a  succession  of  polar- 
ized waves  passing  from  it  to  the  second  grating  B,  and  thence, 
since  the  bars  of  B  are  also  arranged  vertically,  to  the  second 
boy.  Had  the  bars  of  the  grating  B  been  arranged  horizontally — 
that  is,  at  right  angles  to  those  of  the  first — it  is  clear  that  the 
polarized  waves  falling  upon  the  second  grating  would  be  stopped 


wP*ciz<: 


Fig.  67. — Rope  Polariscoke  :  Nicols  Crossed. 

as  in  Fig.  67,  but  in  all  intermediate  positions  of  these  bars, 
between  the  vertical  and  horizontal,  the  waves  from  A  would  be 
partly  stopped  and  partly  transmitted  with  the  plane  of  vibration 
changed.  Finally,  were  it  required  to  determine  the  plane  in 
which  the  vibrations  were  taking  place  in  the  waves  falling  upon 
B,  it  would  only  be  necessary  to  rotate  this  grating  into  such  a 
position  that  no  waves  were  transmitted.  The  bars  would  then 
be  at  right  angles  to  the  sought-for  direction.  In  this  way  the 
grating  B  could  be  used  to  test  the  polarized  condition  of  the 
waves  falling  upon  it — i.e.,  as  an  analyzer. 

Polarized  Light. — Ordinary  light  may  be  looked  upon  as 
consisting  of  waves  transmitted  through  the  ether  of  space  by 
the  to-and-fro  motions  of  the  particles  of  that  ether  in  all  direc- 


222  MODERN  MICROSCOPY 

tions  across  the  line  of  march  of  the  waves  ;  but  when  such  a 
beam  of  light  falls  upon  a  nicol  prism,  it  is  polarized — that  is, 
after  passing  through  the  prism,  the  light-waves  are  found  to 
have  their  vibrations  in  a  single  and  constant  plane.  The  nicol 
prism,  therefore,  under  these  circumstances  acts  upon  light- 
waves in  an  analogous  way  to  that  in  which  the  grating  A  acted 
upon  the  rope-waves.  Similarly,  the  polarized  light  produced 
by  the  action  of  one  nicol  prism  being  allowed  to  fall  upon  a 
second  one  similarly  arranged,  will  pass  through  it,  just  as  the 
waves  from  the  grating  A  passed  through  the  grating  B,  when 
similarly  arranged,  as  in  Fig.  66.  The  second  nicol  also  being 
turned  through  a  right  angle  from  the  position  just  described, 
completely  stops  the  light  falling  upon  it  from  the  first  one,  just 
as  the  second  grating  B  stopped  the  waves  from  A  in  the  position 
shown  by  Fig.  67.  Finally,  in  all  intermediate  positions  of  the 
second  nicol  the  light  will  be  more  or  less  completely  transmitted, 
but  with  its  plane  of  vibration  changed.  To  sum  up,  therefore, 
we  may  say  that — 

1.  In  ordinary  or  common  light,  such  as  sunlight,  the  vibra- 
tions of  the  ether  particles  concerned  in  the  transmission  of  the 
light-waves  take  place  in  every  possible  direction  across  the 
direction  in  which  the  light  is  moving. 

2.  A  nicol  prism  (or  its  equivalent)  is  a  kind  of  optical  grating 
which  reduces  the  vibrations  of  ordinary  incident  light  to  a 
single  direction  or  plane  only,  and  is  thus  said  to  act  as  a 
polarizer.* 

3.  A  nicol  prism,  when  polarized  light  falls  upon  it,  transmits 
it  in  general  more  or  less  completely  with  its  plane  of  vibration 
changed,  but  in  the  particular  case  when  it  is  so  arranged  that 
the  direction  in  which  it  allows  ether  vibrations  to  take  place  is 
at  right  angles  to  the  direction  in  which  the  vibrations  in  the 
incident  polarized  light  are  taking  place,  it  stops  the  incident 
light  altogether,  and  thus  acts  as  an  analyzer. 

Double  Refraction. — The  peculiar  optical  property  possessed 
by  most  crystals  in  virtue  of  which  their  examination  and  dif- 
ferentiation in  a  penological   microscope   becomes   possible  is 

*  This  action  of  a  nicol  prism  must  not  be  confounded  with  that  of  the 
grating  employed  in  spectrum  analysis,  which  acts,  of  course,  in  an  entirely 
different  way. 


THE'  USE  OF  THE  PETROLOGICAL  MICROSCOPE     223 


3 


known  as  double  refraction,  and  is  due  to  what  might  very 
appropriately  be  called  '  optical  grain.'  A  piece  of  wood  has  a 
grain  which  is  usually,  it  is  true,  apparent  to  the  eye ;  but  even 
in  the  absence  of  such  evidence  the  fact  could  soon  be  determined 
experimentally,  as  by  attempting  to  split  the  wood  with  a  hatchet 
in  various  directions.  Rock  crystal,  the  clear  and  transparent 
variety  of  quartz,  sometimes  takes  the  form  of  long  prisms  of 
hexagonal  cross-section,  as  shown  by  Fig.  68.  Now,  it  is  found 
that  if  a  slice  be  cut  lengthwise  from  such  a  crystal,  as  indicated 
at  AA,  and  smeared  with  wax  on  one  of  its  faces,  the  application 
of  heat  to  a  point  on  that  face  will  cause  the  wax  to  melt.  The 
area  over  which  the  melting  occurs  will  not,  however — as  in  the 
case  of  glass,  for  example — take  a  circular  form,  but  an  elliptical 
one,  with  the  major  axis 
of  the  ellipse  parallel  to 
the  geometrical  axis  of  the 
crystal.  This  experiment 
shows  that  heat  is  trans- 
mitted more  rapidly  along 
the  crystal  than  across  it. 
In  a  slice  cut  across  the 
axis,  as  at  BB,  the  melted 
area  takes  a  circular  form, 
showing    that    across    the 

crystal  heat  is  transmitted  equally  in  all  directions.  Rock 
crystal  is  thus  shown  to  be  possessed  of  a  kind  of  grain  running 
in  the  direction  of  its  length.  In  consequence  of  this  grain, 
which  is  optical  as  well  as  thermal,  it  is  found  that  if  a  beam 
of  light  be  allowed  to  fall  normally  upon  the  face  of  such  a 
slice  of  rock  crystal,  the  latter  will  be  found  to  act  as  a  kind 
of  double  grating,  sifting  and  transmitting  the  incident  light 
as  two  beams,  in  one  of  which  the  vibrations  take  place  only 
in  the  direction  of  the  length  of  the  crystal,  and  in  the  other 
beam  across  it  only.  A  nicol  prism,  it  may  be  remarked,  is 
a  crystal  of  Iceland  spar  in  which  one  of  the  two  beams  just 
referred  to  is  thrown  out  of  the  way  by  reflection  on  an  artificial 
interface.  A  beam  of  light  passed  along  the  axis  of  a  quartz 
crystal  is  not  split  up  into  two  beams,  but  in  any  other  direc- 
tion it  is  ;  hence  rock  crystal  is  what  is  known  as  a  uniaxial 


Fig.  68. — Crystals  of  Quartz. 


224 


MODE  EN  MICROSCOPY 


crystal.  In  other  crystals,  as  mica  and  selenite,  there  are  two 
directions  in  which  light  passes  without  being  split  up  ;  these 
are  therefore  known  as  biaxial  crystals. 

Now  the  two  polarized  beams  into  which  light  is,  in  general, 
split  up  on  its  passage  through  a  crystal,  travel  with  different 
speeds,  and  in  most  cases  in  slightly  different  directions — hence 
the  term  '  double  refracting.'  In  the  case  of  rock  crystal,  for 
example,  the  beam  in  which  the  vibrations  occur  along  the  axis 
of  the  crystal  travels  more  slowly  than  the  beam  in  which  the 
vibrations  occur  across  it.*  Let  such  a  slice  of  rock  crystal  be 
placed  between  crossed  nicols,  and  arranged  with  its  axis 
inclined  to  the  vibration-directions  P  and  A  of  the   polarizer 

and  analyzer  respectively,  as 
shown  by  Fig.  69.  Then 
polarized  light  coming  upwards 
from  the  polarizer  towards  the 
observer  will  be  resolved  by 
the  crystal  into  two  beams  with 
vibrations  in  rectangular  planes 
and  passing  with  different  speeds. 
These  two  beams,  falling  upon 
the  analyzer,  will  each  again  be 
resolved  into  two  beams — one 
with  horizontal  and  the  other 
with  vertical  vibrations.  The  first  of  these  will  pass  the  analyzer, 
the  second  will  be  stopped.  But  further  than  this,  the  light  which 
passes  the  analyzer  and  emerges  as  a  single  polarized  beam  is 
compounded  of  two  beams  of  equal  intensities,  one  of  which 
passed  through  the  crystal  with  vibrations  along  aa,  and  the 
other  with  vibrations  along  bb /  and,  since  these  beams  travelled 
at  different  speeds,  it  follows  that  upon  being  compounded  by 
an  analyzer  the  waves  in  one  will  be  more  or  less  out  of  step 
with  those  in  the  other,  with  the  result  that  interference  will 
take  place,  and  the  waves  corresponding  to  any  colour  trans- 

*  This  statement  is  made  upon  the  usual  assumption  that  the  vibrations  of 
the  ether  take  place  in  a  direction  at  right  angles  to  the  plane  of  polarization. 
It  should  also  be  remembered  that  light  polarized  by  reflection  at  a  plane 
glass  surface  is  defined  as  being  polarized  in  the  plane  of  reflection.  It 
follows  therefore  from  the  given  assumption  and  the  definition  that  the 
vibrations  in  the  polarized  reflected  light  are  executed  in  directions  parallel 
to  the  surface  of  the  glass. 


Fig.  69.— Section  of  Quartz  cut 
Parallel  to  Axis. 


THE  USE  OF  THE  PETROLOGICAL  MICROSCOPE     225 

mitted  in  opposite  phase  by  the  two  paths  will  be  destroyed, 
leaving  the  transmitted  beam  coloured.  Thus,  when  the  crystal 
is  of  such  a  thickness  that  the  waves  by  the  slow  path  emerge 
550  micromillimetres  (550  fjifi)  behind  those  passing  by  the  fast 
path,  green  light  with  a  wave-length  equal  to  this  quantity  will 
be  cut  out,  leaving  the  transmitted  beam  of  the  complementary 
colour  red.  As  shown  by  Fig.  69,  an  amplitude  oh  in  the  polar- 
izer is,  after  being  resolved  along  each  of  the  two  vibration- 
directions  in  the  crystal  and  the  vibration-direction  (horizontal) 
of  the  analyzer,  represented  by  two  equal  amplitudes  oa  and  oaf. 
Different  colours  produced  in  this  and  analogous  ways  have  been 
very  carefully  studied,  and  are  set  out  in  the  following  table. 
This  succession  of  colours  would  be  produced  by  a  wedge  of 
selenite  with  its  vibration-directions  adjusted  diagonally  as  in 
Fig.  69,  between  crossed  nicols,  the  thickness  of  the  wedge 
increasing  from  nothing  up  to  about  0#2  mm.     The  first  column 

NEWTON'S  COLOUR  SCALE  ACCORDING  TO  QUINCKE. 


Retarda- 
tion in 
Micromilli- 
metres. 

Interference  Colour 

between  Crossed 

Nicols. 

Order. 

Retarda- 
tion in 
Micromilli- 
metres. 

Interference  Colour 

between  Crossed 

Xicols. 

Order. 

\t 

o 

CD 

m 

0 
40 
.     97 
158 
218 
234 
259 
267 
275 
281 
306 
332 
430 
505 
536 
551 

Black. 
Iron-grey. 
Lavender-grey. 
Greyish-blue. 
Clearer  grey. 
Greenish-white. 
Almost  pure  white. 
Yellowish- white. 
Pale  straw- yellow. 
Straw-yellow. 
Light  yellow. 
Bright  yellow. 
Brownish-yellow. 
Reddish-orange. 
Red. 
Deep  red. 

ft 
/ 

843 
866 
910 
948 
998 
1,101 

1,128 
1,151 
1,258 
1,334 
1,376 
1,426 
1,495 
1,534 
1,621 

Yellowish  -  green . 

Greenish-yellow. 

Pure  yellow. 

Orange. 

Bright  orange-red. 

Dark  violet-red. 

Light  bluish-violet. 
Indigo. 

Greenish- blue. 
Sea-green. 
Brilliant  green. 
Greenish-yellow. 
Flesh  colour. 
Carmine-red. 
Dull  purple. 

A 

H 

- 

565 
575 

589 
664 
728 
747 
826 

Purple. 

Violet. 

Indigo. 

Blue  (sky-blue). 

Greenish-blue. 

Green. 

Lighter  green. 

H 

>   3 

m 
J 

1,652 
1,682 
1,711 
1,744 
1,811 
1,927 
2,007 

Violet-grey. 

Greyish-blue. 

Dull  sea-green. 

Bluish-green. 

Light  green. 

Light  greenish-grey. 

Whitish- grey. 

xi 

-  0 

1    O 

ft 

15 


226 


MODEKN  MICROSCOPY 


gives  the  retardation— ?'.<>.,  the  distance  in  micromillimetres 
which  one  beam  emerges  behind  the  other  after  passing  through 
the  crystal — whilst  the  second  column  gives  the  colour  correspond- 
ing to  such  retardation. 

Rotary  Polarization. — Certain  crystals  possess  in  a  certain 
direction  the  remarkable  power  of  rotating  or  twisting  the  plane 
of  vibration  of  a  polarized  beam  passing  through  them.  Thus, 
if  a  slice  of  rock  crystal  1  mm.  thick,  cut  from  the  crystal  normal 
to  the  axis  as  at  BB,  Fig.  68,  be  placed  between  crossed  nicols 
in  white  light,  it  is  found  that  the  analyzer  no  longer  stops  the 
light,  and  that  no  position  can  be  found  for  it  in  which  it  does 

stop  it.  In  the  case  of  sodium 
light,  however,  it  is  found  that 
a  rotation  of  the  analyzer  from 
its  crossed  position  with  respect 
^  to  the  polarizer  through  an 
— y  angle  of  22°  again  establishes 
darkness.  In  some  instances 
this  necessary  rotation  has  to 
be  made  in  the  direction  of  the 
hands  of  a  clock,  from  the 
observer's  point  of  view,  about 
the  direction  in  which  the  light 
is  passing  to  the  eye,  whilst  in 
others  it  has  to  be  made  in  the 
opposite  direction.  In  the  first 
case  the  rotary  polarization  is  said  to  be  right-handed,  and  in  the 
second  case  left-handed.  If,  then,  a  beam  of  polarized  sodium  light, 
in  which  the  vibrations  are  vertical,  is  allowed  to  pass  along  the 
axis  of  a  quartz  crystal,  the  plane  of  vibration  will  not  remain 
vertical,  but  will  be  gradually  rotated  or  twisted  about  that  axis 
at  the  rate  of  22°  per  mm.  of  length  of  the  crystal.  For  different 
colours  in  the  incident  polarized  white  light,  the  rate  of  turning 
is  different.  Red  light,  for  example,  has  its  plane  of  vibration 
twisted  at  the  rate  of  13°  per  mm.,  whilst  blue,  on  the  other 
hand,  is  twisted  through  as  many  as  33°  in  the  same  length. 
Thus,  if  Fig.  70  be  taken  to  represent  this  action  in  a  slice 
of  right-handed  quartz  of  the  specified  thickness  in  incident 
white  light,  polarized  with  its  vibrations  along  the  line  PP,  and 


Fig.  70.  —  Section  of  Quartz, 
Normal  to  Axis,  to  show  Rotary 
Polarization. 


THE  USE  OF  THE  PETEOLOGICAL  MICROSCOPE     227 

passing  upwards  to  the  eye,  the  vibration  planes  for  the  red, 
yellow,  and  blue  will  be  twisted  into  the  directions  RR,  YY,  and 
BB  respectively,  through  angles  a,  /3,  and  7,  equal  to  13°,  22°,  and 
33°  respectively.  The  analyzer,  therefore,  set  originally  parallel 
with  the  polarizer  and  rotated  in  the  direction  of  the  hands  of 
a  clock,  would  allow  in  succession  the  colours  red,  yellow,  and 
blue  to  pass  in  predominance  to  the  eye,  so  that  the  crystal 
would  appear  to  change  in  colour  during  the  rotation  of  the 
analyzer. 


Optical  Adjuncts  for  the  Petrological  Microscope. 

Crystallographic  determinations  are  very  much  facilitated  by 
the  employment  of  a  number  of  optical  adjuncts,  the  more 
indispensable  of  which  are  set  out  below. 

Mica  Quarter- Wave  Plate. — This  consists  of  a  cleavage  plate 
of  mica  of  such  a  thickness  that  sodium  light,  with  a  wave- 
length of  589  p/A,  in  passing  through  it,  along  one  of  the  two 
vibration-directions  possible  in  a  double-refracting  crystal, 
emerges  a  quarter  of  a  wave-length  behind  that  passing  through 
by  the  other  rectangular  vibration-direction.  These  vibration- 
directions  are  often,  therefore,  referred  to  as  the  '  fast '  and 
1  slow '  directions  to  differentiate  them.  If  a  quarter- wave  mica 
then  be  placed  on  a  crystal  section,  so  that  similar  directions 
in  the  two  sections  are  parallel  to  one  another,  the  effect  is  the 
same  as  that  which  would  have  been  obtained  by  increasing  the 
thickness  of  the  crystal  section,  and  if  the  latter  should  be 
between  crossed  nicols  with  its  vibration -direction  inclined  at 
45°  to  the  vibration-directions  of  the  nicols,  its  colour  would 
rise  in  Newton's  scale — i.e.,  correspond  to  a  greater  retardation. 
By  superposing  the  mica  with  its  fast  direction  parallel  to  the 
slow  direction  of  the  crystal  section,  the  effect  would  be  the  same 
as  that  which  would  have  been  obtained  by  decreasing  the  thick- 
ness of  the  crystal  section,  so  that  in  this  case  the  colour  would 
descend  in  Newton's  scale — i.e.,  correspond  to  a  less  retardation. 
By  the  use  of  a  quarter-wave  mica  in  this  way  the  fast  and 
slow  directions  of  crystal  sections  are  differentiated. 

Selenite  or  Gypsum  Plate. — When  it  is  required  to  differ- 
entiate the  fast  and  slow  directions  of   a  crjstal   section  with 


228 


MODEKN  MICROSCOPY 


small  bi-refracting  power,  a  selenite  plate  of  such  a  thickness 
as  to  give  a  rose  colour  between  crossed  nicols  is  employed. 
Superposed  upon  a  crystal  section  with  similar  directions  parallel, 
the  colour  changes  to  blue,  whilst  the  placing  of  fast  on  slow 
changes  the  colour  to  red. 

Klein  Quartz  Plate. — A  plate  of  quartz  3*75  mm.  thick,  and 
cut  at  right  angles  to  the  axis  of  the  crystal,  gives  between 
crossed  nicols,  and  in  virtue  of  its  rotary  polarizing  power,  a 
purple  colour.  In  this  case  the  orientation  of  the  plate  does 
not  affect  either  the  intensity  or  the  colour  of  the 
light  transmitted. 

Bertrand  Plate. — This  plate  is  made  up  of  four 
quadrantal  sectors  of  alternately  right-  and  left- 
hand  quartz,  cut  at  right  angles  to  the  axis  of  the 
crystal,  and  2 '5  mm.  thick. 

Fedorow  Mica-Steps.  —  This  is  built  up  by 
superposing  some  sixteen  strips  of  quarter-wave  mica,  all  with 
similar  directions  parallel,  in  such  a  way  that  each  strip  is 
about   2    mm.    shorter   than    the    one    immediately    below    it. 


Fig.  71. 

Bertrand 
Plate. 


I       |/V 

_i — i — i — i — i — i — i 

i  -  ;    i        ii 

i    i     -i         ^^j 


Fig.  72.— Mica-Steps. 


Sixteen  steps  are  thus  formed,  which  effect  in  succession 
retardations,  increasing  by  a  quarter-wave  at  each  step,  com- 
mencing with  a  quarter-wave,  and  finishing  with  four  waves. 

Quartz  Wedge. — A  thin  slice  of  quartz  cut  parallel  to  the 
crystallographic  axis  is  ground  into  the  form  of  a  thin  wedge. 
Could  such  a  wedge  be  ground  to  an  infinitely  thin  edge,  it 
would  give  at  this  edge,  when  oriented  with  its  vibration- 
directions  in  diagonal  adjustment  between  crossed  nicols,  the 
black  of  Newton's  colour  scale,  followed  by  all  the  colours  of  the 
scale  in  ascending  order  in  passing  along  the  wedge  to  the  thick 
end.  Wedges  giving  the  first  six  orders  of  Newton's  scale,  or 
some  smaller  number  if  required,  are  thus  made.     To  avoid  the 


THE  USE  OF  THE  PETEOLOGICAL  MICROSCOPE     229 

necessity  for  grinding  a  very  thin  edge,  the  quartz  plate  A  from 
which  the  wedge  is  to  be  made,  is  cemented  to  a  thin  plate  of 
selenite  B,  with  the  fast  direction  of  one  parallel  to  the  slow 
direction  of  the  other.  By  this  device  it  is  made  easy  to  get 
the  starting  black  at  the  thin  end  of  the  quartz  without  reducing 
that  end  to  a  less  thickness  than  the  selenite  foundation-plate 
possesses.  Sometimes  this  foundation-plate  is  made  to  give 
the  sensitive  rose  tint  of  the  first  or  second  order,  and  is  made 
to  project  for  a  short  distance  beyond  the  thin  end  of  the  quartz. 
The  wedge  should  carry  a  scale  along  its  length,  from  which  the 
retardation  in  micromillimetres  at  any  point  of  the  wedge,  and 
for  any  given  tint,  can  be  determined. 

It  will  be  found  that  the  optical  adjuncts  referred  to  above, 
as  produced  by  different  makers,  are  unfortunately  not  uniformly 


3 

A 

1 
'       1 

1         1 

1 

~1 

r~ 

i — ' 

— ' 

- 

i 
i 

Fig.  73. — Quartz  Wedge. 

mounted  as  regards  the  orientation  of  their  vibration-directions 
with  respect  to  the  length  of  the  plate  or  wedge.  Sometimes 
the  fast  direction  is  coincident  with  the  length  of  the  plate, 
sometimes  it  is  across  it,  and  sometimes  it  will  be  found  inclined 
at  45°  to  the  length.  The  last  disposition  has  the  advantage 
that  by  inverting  the  plate  in  the  cross  slot  in  the  tube  of  the 
microscope,  the  optical  superposition  of  fast  on  fast  can  be 
changed  for  slow  on  fast  without  any  difficulty. 

The  Construction  of  the  Petrological  Microscope.— Fig.  74 
shows  a  first-class  modern  instrument.  The  sub-stage  polarizer 
is  associated  with  a  triple  condensing  system  for  convergent 
light,  two  lenses  of  which  can  be  turned  to  one  side  when  plane- 
polarized  light  is  required.  The  stage  is  rotatable,  and 
graduated  to  read  to  a  tenth  of  a  degree  with  the  help  of  a 
vernier.  The  collar  to  which  the  objective  is  secured  is  fitted 
with  centring  screws,  and  is  made  with  a  slot  into  which  the 
usual  mica  and  gypsum  compensators  can  be  introduced.     The 


Fig.  74. — A  Modern  Petrological  Microscope. 


THE  USE  OF  THE  PETEOLOGICAL  MICKOSCOPE     231 

analyzer  fitted  above  the  objective  can  be  pushed  radially  into 
and  out  of  action,  and  it  can  further  be  rotated,  when  required, 
through  an  angle  of  90°  or  less  about  the  axis  of  the  microscope 
body-tube,  and  clamped  in  position.  The  Bertrand  lens  slides 
into  position  near  the  middle  of  the  length  of  the  tube.  The 
draw -tube  is  operated  by  a  rack  and  pinion,  and  an  auxiliary 
analyzer  with  divided  circle  and  a  slot  for  compensators  may  be 
fitted  over  the  eyepiece.  The  fine  adjustment  head  is  graduated 
to  read  directly  to  a  thousandth  of  a  millimetre. 

Preliminary  Adjustment  of  a  Penological  Microscope. — 
Before  any  work  is  done  the  following  adjustments  should  be 
carefully  tested,  and,  if  necessary,  made : 

1.  Centring  of  the  objective. 

2.  Kectangularity  of  the  cross-wires  in  the  eyepiece. 

3.  Kectangularity  of  the  vibration  planes  of  the  two  nicol 
prisms. 

4.  Parallelism  of  the  cross-wires  to  the  vibration  planes  of  the 
two  nicol  prisms  when  the  latter  are  crossed. 

Centring  of  the  Objective. — This  is  done  in  microscopes  of 
the  usual  type  by  the  manipulation  of  two  radial  set-screws 
acting  against  the  collar  in  which  the  objective  is  secured.  A 
slide  should  be  placed  upon  the  stage,  and  a  prominent  point 
or  feature  of  it  adjusted  to  the  intersection  of  the  cross-wires. 
Upon  a  complete  rotation  of  the  stage  the  point  selected  will 
describe  a  small  circle  in  the  field  of  view ;  consequently,  when 
the  stage  has  been  rotated  through  180°  only,  the  point  selected 
will  have  its  maximum  displacement  from  the  intersection  of 
the  cross-wires.  Stop  the  rotation,  therefore,  at  this  point,  and 
turn  the  adjusting  screws  so  as  to  move  apparently  the  inter- 
section of  the  wires  half-way  towards  the  selected  point.  The 
adjustment  will  now  be  found  to  be  very  nearly  correct.  Eepeat 
the  operation  until  it  is  quite  so. 

Rectangularity  of  the  Cross-Wires  in  the  Eyepiece. — 
Place  a  slide  with  a  fine  straight  line  ruled  upon  it  on  the  stage, 
and  adjust  it  until  the  projected  image  of  the  line  coincides  with 
one  of  the  cross-wires.  Note  the  angular  position  of  the  stage, 
and  rotate  it  carefully  through  a  right  angle.  The  projected 
image  of  the  line  should  now  be  parallel  to  the  second  cross- 
wire.     If  the  centring  of  the  objective  has  been  first  effected  as 


232  MODEEN  MICROSCOPY 

above,  the  projected  image  in  the  second  case  will  coincide  with 
the  second  cross-wire. 

Rectangularity  of  the  Vibration  Planes  of  the  Nicol 
Prisms. — Darkness  of  the  field  is  not  a  sufficiently  delicate  test 
for  this  adjustment,  but,  if  the  necessary  adjunct — a  Bertrand 
quarter-quartz  plate — is  not  available  for  a  more  delicate  adjust- 
ment, darkness  should  be  obtained  a  number  of  times  by 
rotation  of  the  polarizer  alternately  in  opposite  directions.  The 
mean  of  the  various  readings  should  be  taken  as  the  true  one. 
To  obtain  a  better  result,  place  a  Bertrand  plate  upon  the  stage, 
set  the  polarizer  to  zero,  and  slide  the  analyzer  into  position. 
If  the  vibration  planes  of  the  two  nicols  are  accurately  at  right 
angles  to  one  another,  the  quadrants  of  the  Bertrand  plate  will 
appear  to  have  the  same  tint.  Otherwise,  the  colours  of  adjacent 
quadrants  will  not  match,  in  which  event  the  polarizer  must  be 
rotated  until  they  do  match,  when  the  necessary  zero  correction 
should  be  read  off  on  the  polarizer.  If  the  polarizer  is  not  fully 
graduated  so  as  to  allow  of  this  being  done,  a  fine  vertical  line 
should  be  drawn  across  the  junction  of  the  polarizer  mount  and 
the  sleeve  into  which  it  is  pushed.  Better  still,  if  possible,  the 
polarizing  prism  should  be  rotated  in  its  mount,  until  the  latter 
being  at  zero,  the  adjustment  is  correct. 

Parallelism  of  the  Cross-Wires  to  the  Vibration  Planes  of 
the  Crossed  Nicols. — A  needle-shaped  crystal  such  as  anhydrite 
(anhydrous  sulphate  of  calcium,  crystallizing  in  the  orthorhombic 
system),  in  which  one  of  the  directions  of  extinction  is  parallel 
to  the  long  edges  of  the  crystal,  should  be  placed  on  the  stage 
between  crossed  nicols,  and  rotated  until  extinction  is  obtained. 
Pull  the  analyzer  out,  when  the  crystal  should  be  seen  ranged 
parallel  to  one  of  the  cross- wires.  Turn  the  crystal  over  on  the 
stage  and  repeat. 

Examination  and  Identification  of  the  Crystalline  Con- 
stituents of  Rock  Sections. 

As  this  chapter  does  not  profess  to  be  anything  more  than  an 
introduction  to  the  use  of  the  penological  microscope,  no 
attempt  will  be  made  to  describe  the  complete  and  systematic 
examination  usually  made  of  crystal  sections  by  expert  miner- 


THE  USE  OF  THE  PETROLOGICAL  MICROSCOPE     233 

alogists.  Some  of  the  simpler  determinations  only  will  be 
indicated.  Suppose  that  an  angular  crystal  which  lights  up  and 
darkens  between  crossed  nicols  upon  rotation  of  the  stage  is 
to  be  examined,  we  could  proceed  to  determine  (1)  the  angles 
between  the  sides  of  the  crystal ;  (2)  the  angular  positions  of  the 
extinction-directions  with  respect  to  the  sides  ;  (3)  the  differentia- 
tion of  these  extinction-directions  into  fast  and  slow  ;  and  (4)  the 
retardation  of  the  section. 

To  Measure  the  Plane  Angles  of  a  Crystal  Section. — 
Neither  nicol  is  necessary.  Adjust  the  section  on  the  stage 
until  one  of  the  sides  of  the  section  is  projected  along  a  cross- 
wire.  Take  the  stage  (angular)  reading.  Eotate  the  stage  until 
the  second  side  is  brought  into  alignment  with  the  same  cross- 
wire.  Take  a  second  stage  reading.  The  difference  between 
these  two  readings  gives  the  desired  angle. 

To  Determine  the  Angular  Positions  of  the  Extinction- 
Directions. — Cross  the  nicols  and  adjust  the  section  until  a 
side  of  the  crystal  coincides  with  a  cross-wire.  Take  the 
stage  reading.  Rotate  the  stage  to  extinction.  Take  the 
stage  reading  again.  The  difference  is  the  desired  angle. 
Repeat,  and  take  the  mean  value  of  the  results.  To  make  a 
more  accurate  determination,  advantage  is  taken  of  the  fact 
that  the  four  sectors  of  a  Bertrand  quarter  -  quartz  plate, 
placed  in  the  eyepiece  between  accurately  crossed  nicols,  will 
appear  of  one  uniform  tint  whenever  a  bi-refracting  plate  on 
the  stage  is  rotated  into  such  a  position  that  its  vibration - 
directions  are  parallel  to  those  of  the  crossed  nicols.  This 
method  necessitates  the  employment  of  an  analyzer  above  the 
eyepiece. 

To  Differentiate  the  Extinction- Directions. — This  may  be 
done  by  the  use  of  a  mica  quarter-wave  plate  in  the  way  already 
described.  When,  however,  the  bi-refracting  power  of  the  section 
being  examined  is  very  small,  it  is  better  to  employ  a  sensitive 
selenite  plate  (so-called  red  of  the  first  order).  When  this  is 
introduced  into  the  cross-slot  just  over  the  objective,  so  that 
its  fast  direction  is  parallel  to  the  fast  direction  of  the  crystal 
section,  the  rose  colour  changes  to  a  blue ;  whilst  when  the  fast 
direction  is  superposed  on  the  slow  of  the  section,  the  colour 
becomes  a  bright  red. 


234 


MODEEN  MICEOSCOPY 


Retardation.— This  quantity  is  most  simply  determined  by 
the  use  of  the  quartz  wedge  or  the  mica-step  compensator. 
Unfortunately,  however,  these  cannot  be  used  satisfactorily  in 
the  usual  slot  over  the  objective,  because  in  neither  case,  in  the 
final  adjustment,  is  one  thickness  only  of  the  compensator 
operative.  In  the  mica-step,  for  example,  two  or  more  steps 
must  be  interposed  in  the  path  of  the  light  rays  proceeding  from 
the  objective  to  form  the  image  in  the  eyepiece,  whilst  in  the 
case  of  the  quartz  wedge  quite  an  appreciable  fraction  of  the 
total  length  must  be  interposed.  Further,  in  the  latter  case, 
the  retardation  scale  cannot  be  used  since  it  is  not  in  focus. 
When  a  very  low-power  objective  is  sufficient,  the  compensators 
can  be  superposed  on  the  section  on  the  stage,  and  the  retarda- 
tion determined   directly  by  the  position,  in  the  case   of   the 


Fig.  75. — The  Action  of  a  Convergent  System. 


quartz  wedge,  of  the  black  band  on  the  retardation  scale  when 
the  fast  direction  of  the  wedge  is  superposed  on  the  slow  direc- 
tion of  the  section.  In  the  case  of  the  mica-step  the  retardation 
may  be  equal  to  that  of  an  integral  number  of  steps,  but  more 
generally  it  falls  between  two  of  these,  and  an  estimate  of  its 
value  has  to  be  made.  These  compensators  should  always,  if 
possible,  be  used  in  the  focal  plane  of  the  eyepiece.  In  that 
event,  of  course,  the  usual  analyzer  must  be  thrown  out  of  action 
and  one  placed  instead  over  the  eyepiece. 

Examination  in  Convergent  Light. — To  understand  the 
optical  action  which  is  taking  place  in  the  microscope  when  it 
is  being  employed  for  the  examination  of  sections  in  convergent 
light,  it  will  be  better  to  consider  first  the  simple  case  (Fig.  75), 
in  which  a  plate  of  bi-refracting  crystal  A  cut  at  right  angles  to 
the  axis — a  plate  of  calcite,  say — is  interposed  between  two  nearly 


THE  USE  OF  THE  PETROLOGICAL  MICROSCOPE     235 


hemispherical  lenses,  B  and  C.  Further,  let 
plane-polarized  light,  no  matter  how  produced, 
start  from  the  point  b,  on  the  axis  of  the  system 
and  in  the  principal  focal  plane  F  of  the  lens  B, 
and  passing  through  the  lens  B,  plate  A,  and 
lens  C,  be  brought  to  a  focus  by  the  latter  at  the 
point  b'  in  the  focal  plane  F'  of  the  lens  C, 
Light  from  points  a  and  c  will  similarly  be 
brought  to  a  focus  in  points  a  and  c  respectively. 
Now,  it  will  be  observed  that  in  each  of  these 
three  cases  the  light  that  actually  passes  through 
the  plate  A  is  in  the  form  of  parallel  rays,  but 
that  the  inclination  which  any  particular  bundle 
of  parallel  rays  makes  with  the  axis  of  the  crystal 
— the  line  bb' — depends  upon  the  distance  be  or 
ba.  In  the  plane  F',  therefore,  it  follows  that  the 
light  falling  in  the  circular  line  struck  with  a 
radius  b'c',  around  the  point  b',  is  light,  the  whole 
of  which  has  passed  through  the  crystal  plate  at 
the  same  angle  of  inclination  to  the  axis.  In  the 
plane  F',  therefore,  we  get  the  familiar  interference 
figure  which,  when  looked  at  through  a  crossed 
nicol,  appears  as  a  number  of  concentric  rainbow- 
tinted  rings,  with  a  black  cross  marking  them  off 
into  quadrants. 

In  the  actual  microscope  the  condenser  B, 
Fig.  76,  functions  as  the  lens  B  of  Fig.  75, 
and  the  objective  C  as  the  lens  C  of  Fig.  75. 
The  interference  image  shown  as  being  focussed 
in  the  upper  focal  plane  of  the  objective,  is  pro- 
jected by  the  Bertrand  lens  D  and  the  field-lens  E 
of  the  eyepiece,  into  the  stop-plane  of  the  latter, 
and  again  by  the  eye-lens  F  on  to  the  retina  of 
the  eye  of  the  observer.  The  analyzer  is  shown 
fitted  between  the  Bertrand  lens  and  the  objective. 
The  eyepiece  and  the  Bertrand  lens  thus  act 
together  as  a  low-power  compound  microscope  to 
magnify  the  figure  in  the  upper  focal  plane  of  Fig.  76.-- -Ray- 
the  objective  C.  In  the  absence  of  the  Bertrand  A  Microscope 
lens  the  eyepiece  projects  the  interference  figure  arranged  for 
into  the  Ramsden  circle,   where   it  can    be   very    light. 


«q 


236  MODERN  MICROSCOPY 

satisfactorily  observed  with  a  powerful  pocket  magnifier.  In 
the  latter  case  a  small  stop  may  be  placed  in  the  stop-plane 
of  the  eyepiece  to  cut  off  all  light  except  that  which  has  passed 
through  the  crystal  under  examination. 

Plate  L,  from  the  atlas  of  the  late  Dr.  Hausewaldt,  shows  the 
interference  figures  in  convergent  sodium  light  and  between 
crossed  nicols  of  sections  of  arragonite — the  first  pair  due  to 
a  specimen  J  mm.  thick,  the  second  pair  due  to  one  of  4  mm. 
thickness. 

Figs.  77  and  79  show  the  figures  when  the  extinction-directions 
of  the  crystal  are  adjusted  parallel  to  those  of  the  nicol ; 
Figs.  78  and  80  when  those  directions  are  adjusted  diagonally. 

The  attention  of  the  reader  desirous  of  further  information  is 
directed  to  a  paper  by  Dr.  John  Evans  in  the  Proceedings  of 
the  Geologists'  Association,  vol.  xxi.,  part  2,  1909,  on  '  The 
Systematic  Examination  of  a  Thin  Section  of  a  Crystal  with  an 
Ordinary  Petrological  Microscope.'  It  is  to  be  regretted  that 
this  invaluable  brochure  has  not  been  published  in  a  more 
accessible  form.  The  following  textbooks  may  also  be  referred 
to — viz., '  Traite  de  Technique  Mine'ralogique  et  Petrographique,' 
by  Duparc  and  Pearce,  Leipzig,  1909 ;  and  '  Anleitung  zum 
Gebrauch  des  Polarisationsmikroskops,'  by  Weinschenk,  Frei- 
burg im  Breisgau,  1906. 


PLATE    I. 


Fig. 


/  /. 


Fig.  78. 


Fig.  79.  i'"1'-   so- 

Interference  Figures  of  Arragonite.     (Hattsewaldt.] 


[To  faa  p.  236, 


CHAPTEE    XXII 

ROTIFEBA* 

By  C.  F.  EOUSSELET,  F.R.M.S. 

Arranged  in  five  sections  as  follows  : 

1.  Specimens  likely  to  be  found  during  each  month  of  the  year  ; 

2.  Collecting-grounds  near  London  ; 

3.  Methods  of  collecting,  preliminary  examination,  and 
keeping ; 

4.  Apparatus  for  microscopic  examination  ; 

5.  Preserving  and  mounting. 

1.  Specimens  likely  to  be  found  during  each  Month 

of  the  Year. 

January. 

January  is  the  most  severe  month  of  the  year,  and  lakes  and 
ponds  are  often  frozen  over  or  difficult  to  approach.  Micro- 
scopic pond-life,  though  less  abundant  than  in  the  spring  and 
autumn,  is,  nevertheless,  nearly  always  present,  even  under  the 
ice  many  inches  thick.  All  the  following  species  of  rotifers 
have  been  taken  in  January  in  and  near  London ;  but  no 
doubt  a  great  many  more  could  be  found  by  systematic  search  : 
Asjjlanchna  Brightwellii  and  priodonta ;  Anurcea  aculeata  and 
cochlearis ;  Brachionus  pala  and  angularis ;  Notholca  scapha ; 
Euchlanis  deflexa  and  hyalina ;  Rotifer  macrurus  and  vulgaris; 
Polyarthra  platyptera  ;  Synchczta  pectinata,  tremula,  and  oblonga  ; 
Conochilus  unicornis;  Cadopus  porcellus ;  Diaschiza  lacinulata 
and  ventripes ;  Proales  decipiens  and  petromyzon  ;  Diglena  ford- 

*  Originally  published  in  Knowledge,  and  reproduced  by  permission. 

237 


238  MODERN  MICROSCOPY 

pata  ;  Dinocharis  pocillum  ;  Monostyla  cornuta;  Colurus  caudatus; 
Melicerta  ring  ens  ;  Limnias  ceratophyUi  ;  (Ecistes  crystallinus  ; 
Floscularia  cornuta  ;  and  Stephanoceros  eichhornii.  Diaptomus 
and  Cyclops  and  their  larvae  are  abundant,  whilst  Water-fleas  are 
almost  absent.  Aquatic  vegetation  having  died  down,  the  fixed 
forms  of  rotifers  and  Infusoria  should  be  looked  for  on  the  root- 
lets of  trees  growing  near  the  edge  of  the  water.  Floscules  and 
Melicerta  were  once  found  covering  such  rootlets  very  thickly. 
January  seems  to  be  the  time  when  the  males  of  Stephanoceros 
and  other  tube-dwellers  are  found,  and  their  presence  is  often 
betrayed  by  the  thick-shelled,  fertilized,  resting  eggs  in  some  of 
the  tubes,  and  numerous  smaller  male  eggs  in  others. 

February. 

In  the  early  part  of  the  year,  when  the  weather  is  still  cold 
and  ponds  are  covered  with  ice,  some  Infusoria  may  be  found 
in  abundance,  particularly  the  various  species  of  Vorticella — 
Carchesium  polypinum,  Zoothamnium  arbuscula,  Epistylis  flavicans 
— attached  to  submerged  rootlets. 

Rotifera  to  be  looked  for  in  lakes  and  ponds,  particularly 
duck-ponds:  Anurcea  aculeata,  Anurcea  cochlearis,  Asplanchna 
priodonta  and  Brightwellii,  Notholca  scapha,  Polyarthra  platyptera, 
Euchlanis  deflexa,  Synchceta  tremula.  The  water-plants  having 
mostly  died  down,  the  following  fixed  forms  are  found  attached 
on  Anacharis,  or  on  submerged  rootlets  of  plants,  or  on  trees 
growing  near  the  edge  of  ponds  and  lakes  :  Melicerta  ringens, 
Limnias  ceratophyUi,  Stephanoceros  eichhornii,  Floscularia  cornuta, 
and  others  ;  (Ecistes  crystallinus  and  others. 

March. 

The  same  species  as  those  mentioned  for  February  are  still  to 
be  found,  and  in  greater  abundance.  Some  new  Infusoria  will 
have  made  their  appearance,  such  as  Stentor  polymorphic,  which 
will  be  found  covering  the  rootlets  of  Duckweed  and  other  sub- 
merged plants,  Peridinium  tabulation  and  the  free-swimming 
colonies  of  Synura  uvella,  etc.  Then  the  very  minute  and 
beautiful  colonies  of  Collared  Monads,  Godosiga  umbellata,  and 


ROTIFERA  239 

other  species  of  this  group  may  be  looked  for,  attached  to  the 
stems  of  Vorticella  trees. 

All  the  Rotifera  forming  the  winter  fauna  will  become  very 
abundant  in  March,  and  as  the  food-supply  in  minute  Algae  and 
Infusoria  increases,  fresh  species  make  their  appearance  with 
every  rise  of  temperature.  The  following  additional  species 
may  be  looked  for :  Brachionus  angularis ;  Notholca  acuminata, 
spinifera,  and  labis ;  Euchlanis  oropha ;  Dinocharis  pocillwm ; 
Diaschiza  lacinulata ;  Proales  decipiens  and  petronxyzon ;  Mono- 
styla  cornuta;   Diglena  forcipata  ;  Rotifer  vulgaris. 

April. 

All  species  of  Infusoria  and  Rotifera  mentioned  as  occurring 
in  March  are  likely  to  become  more  abundant  in  April,  which  is 
one  of  the  best  months  for  collecting.  The  ponds  are  full  of 
water,  whilst  they  have  become  approachable,  and  Daphnias 
and  Cyclops  have  not  yet  crowded  out  the  rotifers,  as  sometimes 
occurs  later  on.  Volvox  globator  may  be  looked  for,  together 
with  the  little  parasitic  rotifer,  Proales  parasitica,  inside  the 
green  spheres. 

Of  larger  Infusoria,  Bursaria  truncatella,  Charnia  teres,  Amphi- 
leptus  gigas,  and  flagellatus  will  be  found,  and,  of  course,  crowds 
of  Euglena  viridis. 

Of  Rotifera,  Synchceta  pectinata  will  be  abundant,  and 
Asplanchna  priodonta  and  Brightwellii  will  have  made  their 
appearance  in  larger  lakes  and  canals;  also  Brachionus  pala, 
quadratics,  and  Bakeri  ;  Euchlanis  triquetra  and  hyalina ; 
Triartlira  longiseta,  Diaschiza  semiaperta  ;  Rhinops  vitrea, 
Pterodina  patina,  Mastigocerca  bicornis,  and  many  others. 

May. 

All  the  various  pond  organisms  that  die  down  in  winter  and 
in  various  ways  produce  protected  germs  to  tide  over  this,  for 
them,  unsuitable  season,  will  now  have  come  to  life  again  and 
begin  to  multiply  at  an  increasing  rate.  Many  kinds  of  Desmids 
should  be  found  in  shallow,  mossy  pools,  or  along  the  edge  of 
rivulets.     Among  Protophyta  and  Protozoa  the  green  spheres  of 


240  MODERN  MICROSCOPY 

Volvox  globator  will  be  found  in  many  localities  more  or  less 
abundantly,  and  the  various  kinds  of  Acineta  should  be  looked 
for  in  quiet,  undisturbed  waters,  where  many  kinds  of  free- 
swimming  Infusoria  will  also  be  found. 

Of  Rotifera  there  are  few  species  which  may  not  be  found  in 
May.  At  one  excursion  of  the  Quekett  Club  to  Totteridge  in  the 
middle  of  May  forty  different  species  were  obtained.  To  mention 
only  a  few  :  Notops  brachionns,  one  of  the  most  attractive  rotifers, 
will  have  made  its  appearance ;  then  various  kinds  of  Anursea, 
Asplanchna,  Brachionus,  Ccelopus,  Cathypna,  Diaschiza,  Euch- 
lanis,  Furcularia,  Mastigocerca,  Metopidia,  Pterodina,  Synchaeta, 
Scaridium,  Stephanops ;  also  Stephanoceros  eichhomii,  Floscules, 
Melicerta,  and  Limnias  in  abundance. 

On  rootlets  of  trees  growing  near  the  edge  of  ponds  and  lakes 
will  probably  be  found  various  kinds  of  Polyzoa  :  Fredericella 
Sultana,  Paludicella,  and  Plumatella  repens. 


June. 

If  the  months  of  April  and  May  are  abnormally  cold,  pond 
organisms  which  usually  make  their  appearance  in  May  are  likely 
to  be  retarded,  and  will  only  come  on  in  June.  There  are,  however, 
summer  forms  which  hardly  ever  occur  earlier  than  June,  and 
the  most  interesting  of  these  amongst  rotifers  is  Pedalion  minim, 
with  its  six  arthropodous  limbs  ;  Synchceta  sty  lata,  with  its  long- 
spined  floating  eggs,  and  Synchceta  grandis,  the  largest  species 
of  this  genus,  may  also  now  be  looked  for  in  lakes  and  water 
reservoirs,  as  well  as  the  rare  free-swimming  Flosadar i a  pelagica. 
In  the  same  waters  will  be  found  two  free-swimming  colonies  of 
Vorticella  :  Epistylis  rotans  and  Zoothamnium  Umneticum.  In 
June  it  often  happens  that  certain  water-fleas,  Daphnia  and 
Bosmina,  also  Cyclops  and  their  larvae,  increase  to  such  an 
extent  as  to  render  the  existence  of  free-swimming  rotifers 
almost  impossible  in  these  waters,  and  the  latter  consequently 
disappear,  though  they  may  have  been  swarming  a  few  weeks 
earlier.  In  ponds,  however,  where  this  does  not  occur,  rotifers 
of  many  genera  may  be  found,  and  attached  to  submerged  water- 
plants  Lacinularia  socialis  and  Megalotrocha  albo-flavicans  should 
be  looked  for,  whilst  in  reedy  ponds  the  free-swimming  spheres 


ROTIFERA  241 

of  Conochilus  volvox  may  occur.  Mossy  pools,  in  addition  to 
their  special  rotiferous  fauna  of  Philodina,  Callidina,  Adineta, 
Cathypna,  Distyla,  and  Monostyla,  will  also  contain  water-bears 
and  shelled  Rhizopods,  such  as  Diflugia  and  Arcella  and  numerous 
free-swimming  Infusoria.  Polyzoa,  such  as  Plumatclla  repms, 
Freclericella  Sultana,  Lophopus  crystal!  inus,  and  Cristatella  mucedo, 
though  not  common,  should  be  abundant  in  suitable  localities. 

July. 

Collecting  in  July  is  usually  not  so  profitable  as  one  would 
expect,  because  as  a  rule  most  of  the  shallow  ponds  are  dried  up 
by  this  time,  or  have  been  reduced  to  a  muddy  swamp,  and  in 
the  others  Crustaceans,  Cladocera,  and  Cyclops  have  multiplied 
to  such  an  extent  as  to  leave  little  room  for  the  more  interesting 
forms  of  pond-life. 

Pedalion  mirum  should  be  looked  for  in  large  and  small  lakes,  as 
it  will  probably  have  greatly  increased  in  numbers.  The  some- 
what rare  and  very  large  Asplanchna  amphora  and  ebbesbornii,  as 
well  as  Asplanchnopus  myrmcleo,  are  summer  forms  which  occur 
at  this  season.  Other  rotifers  that  appear  in  warm  weather  are  : 
Dinopslongipes;  Triphyllus  lacustris;  Notops  clavulatus;  Scaridium 
eudactilotum  and  longicandum ;  then  the  free-swimming Lac'uudaria 
nutans  and  Conochilus  volvox;  also  the  fixed  Lacinularia  social  is 
and  Megalotrocha,  which  are  found  attached  to  submerged  water- 
plants.  All  these  are  very  beautiful  objects  under  the  microscope, 
but  by  no  means  common. 

Volvox  globator  will  certainly  be  found  in  abundance  now  in 
secluded  ponds,  and  inside  the  green  spheres  the  little  parasitic 
rotifer,  Proales  parasitica,  should  be  looked  for. 

The  Polyzoa,  mentioned  last  month,  will  have  become  more 
abundant  where  they  occur ;  undisturbed  ornamental  lakes  and 
canals  are  the  best  places  to  find  them  in. 

August. 

For  the  collector  of  Cyclops,  Diaptomus,  Water-fleas  and 
aquatic  insect  larvae,  August  is  a  very  capital  month  ;  not  so, 
however,  for  the  collector  of  the  more  interesting  Infusoria  and 
Rotifera,   which    are   usually  quite  crowded   out   by  the   more 

16 


242  MODEEN  MICROSCOPY 

vigorous  Crustaceans  in  the  few  remaining  ponds  and  pools  not 
wholly  dried  up.  In  larger  lakes,  however,  it  is  possible  to  find 
occasionally  a  number  of  interesting  forms,  particularly  free- 
swimming  rotifers,  such  as  Asplanchnapriodonta  and  Brightwellii, 
Synchceta  pectinata,  and  the  rarer  summer  forms,  Synchceta 
stylata  and  grandis.  Where  a  '  green  '  pond  can  be  found  full 
of  the  flagellate  Infusorian  Euglena  viridis,  there  are  usually 
present  also  a  number  of  rotifers,  such  as  Hydatina  senta, 
Eosphora  aurita,  Diglena  biraphis,  etc.,  feeding  on  the  Euglena. 

In  shady  forest  pools,  overgrown  with  Sphagnum,  quite  a 
peculiar  fauna  of  moss-haunting  rotifers  will  be  found,  particu- 
larly various  species  of  Callidina,  Distyla,  Metopidia,  Cathypna, 
in  addition  to  numerous  interesting  Ehizopods  with  shells  of 
various  forms.  In  similar  ponds  the  large  but  very  rare  rotifer, 
Copeus  spicatus,  should  be  looked  for.  Of  other  rotifers  that 
may  be  met  with  in  lakes,  more  or  less  abundantly,  the  following 
can  be  mentioned:  Brachionus  pala ;  Anurcea  aculeata,  brevis- 
pina,  and  hypelasma ;  Dinocharis  pocillum  ;  Euchlanis  triquetra, 
hyalina,  and  oropha ;  Mastigocerca  bicomis,  elongata,  and  stylata; 
Polyartlira  platyptera ;  Synchceta  tremula  and  oblong  a ;  Pedalion 
minim,  and  many  others. 

September. 

In  normal  years  many  of  the  dried-up  ponds  begin  to  fill  up 
again  in  September,  and  become  then  most  prolific  in  infusorian 
and  rotiferous  life,  because  the  disturbing  Crustaceans,  Cyclops, 
and  Cladocera  have  been  to  a  large  extent  eliminated.  But  also 
in  larger  ponds  and  lakes,  which  do  not  dry  up,  the  Crustaceans 
decrease  in  numbers  and  give  the  Eotifera  and  Infusoria  a  fresh 
chance  of  increase.  The  following  free-swimming  forms  may 
often  be  collected  in  immense  numbers :  Asplanchna  priodonta, 
intermedia,  and  Brightwellii;  Triarthra  longiseta ;  Polyarthra 
platyptera;  Synchceta  pectinata,  tremula,  and  oblonga  ;  Anurcea 
aculeata  and  cochlearis  ;  Brachionus  angular  is ;  Pedalion  minim; 
Conochilus  unicornis,  and  the  much  rarer  Floscularia  pelagica. 
Of  the  fixed  forms,  Limnias  ceratophylli  and  annulatus,  Cephalo- 
siplton  limnias,  Lacinularia  socialis,  Melicerta  ring  ens  and  conifera 
should  be  looked  for  on  submerged  water-plants,  such  as  Ana- 
charis,  Ceratophyllum,  Nitella,  and  on  the  rootlets  of  Duckweed. 


ROTIFERA  243 

Polyzoa  such  as  Plumatella,  Lophopus,  Cristatella,  should  be 
found  in  abundance  in  disused  canals  and  backwaters  of  rivers 
and  the  larger  lakes,  from  which  they  could  be  dredged  with  a 
loaded  hook  and  line. 

It  may  be  taken  as  a  general  rule  that  all  the  more  interesting 
forms  of  pond-life  become  more  abundant  in  September,  pro- 
vided only  that  the  weather  is  not  too  hot,  but  tempered  by  re- 
peated showers  to  fill  the  dried-up  ponds  with  a  fresh  supply  of 
rain-water. 

October. 

October  is  one  of  the  best  months  for  the  pond-hunter;  the 
weather  is  cooler,  the  ponds  have  become  filled  with  rain-water 
again,  with  plenty  of  food  material  in  the  shape  of  flagellate 
Infusoria,  and  the  Crustaceans  are  on  the  decline.  In  this 
month  the  greatest  variety  in  species  of  Rotifera  is  usually 
found,  particularly  of  the  smaller  and  rarer  kinds,  and  not  in- 
frequently thirty  to  forty  species  may  be  obtained  in  one  or  two 
small  ponds.  As  a  general  rule  one  cannot  expect  much  variety 
when  a  few  species  are  present  in  excessive  abundance.  The 
following  is  a  list  of  forty-four  species  of  rotifers  actually  col- 
lected on  one  occasion  in  three  ponds  on  October  15,  1898, 
showing  what  may  be  looked  for : 

Floscidaria  regalis,  omata,  cornuta,  ambigua,  edentata,  and 
annulata ;  Limnias  annulatus,  var.  granulosus;  CEcistes  crystal- 
linus ;  Philodina  megalotrocha ;  Rotifer  vulgaris ;  Synclueta 
tremula  and  oblong a  ;  Asplanclma  priodonta ;  Notops  hyptopus ; 
Polyarthra  platyptera ;  Eosphora  aurita;  Furcularia  longiseta, 
sterea,  and  forficula ;  Proales  felis ;  Diglena  biraphis  ;  Mastigo- 
cerca  rattus  and  bicoriiis ;  Ccelopus  porcellus  and  tenuior ;  Batttdus 
bicornis;  Diaschiza  exigua;  Distyla  flexilis  ;  Monostyla  lunaris  ; 
Dinocharis  pocillum ;  Stephanops  lamellar  is ;  Cathypna  luna; 
Euchlanis  oropha;  Metopidia  acuminata;  Brachionus  angularis 
and  Bakeri;  Pompholyx  sulcata;  Notholca  labis  and  scapha; 
Anurcea  acideata,  cochlearis,  tecta,  hypelasma,  and  stipitala. 

On  the  other  hand,  the  various  kinds  of  rotifers  known  as 
summer  forms  will  now  have  disappeared.  Pedalion  mirum  is 
such  a  form,  which  may  occasionally  still  be  seen  during  a  warm 
October,  but  is  then  usually  very  scarce  or  absent. 


244  MODERN  MICROSCOPY 

November. 

With  the  advent  of  November,  pond-life  all  round  becomes 
less  abundant,  and  fewer  species  are  to  be  met  with.  By  degrees 
many  of  the  water-plants  die  down,  and  the  fauna  is  reduced  to 
such  forms  as  can  subsist  through  the  winter.  Those  animals 
which  cannot  do  this,  such  as  Polyzoa,  Daphnia,  some  Rotifera, 
etc.,  have  by  this  time  produced  so-called  winter  eggs  or  resting 
germs.  The  winter  fauna,  however,  is  much  more  numerous 
than  is  usually  assumed.  Among  rotifers,  several  species  of 
Synchseta — S.  pectinata,  tremula,  and  oblonga — seem  to  like 
the  winter  as  well  as  the  summer :  Asplanchna  priodonta, 
Anurcm  aculeata,  Polyarthra  platyptera,  Rotifer  vulgaris, 
Euchlanis  deflexa,  Triarthra  longiseta,  Brachionus  angidaris, 
Conocliilus  unicornis,  Diglena  forcipata,  Diaschiza  lacinulata 
and  ramphigera,  Dinocharis  tetractis,  and  others.  Among  the 
Infusoria,  the  Yorticella  in  particular  seem  to  like  the  cold 
season,  and  a  number  of  different  species,  and  often  large 
colonies  can  be  found  attached  to  submerged  rootlets  of  trees 
growing  near  the  edge  of  the  water.  Attached  to  the  fine  stems 
of  Carchesium,  Zoothamnium,  and  other  stalked  colonies  of 
Vorticella,  the  very  much  more  minute  but  beautiful  colonies 
of  Collared  Monads,  Codosiga,  etc.,  are  often  found,  and  deserve 
a  good  look  with  the  higher  powers. 

In  canals  and  lakes  where  Cristatella  has  been  abundant 
during  the  summer,  their  spiny  stadoblasts  may  now  be  found 
liberated  and  often  in  large  masses  floating  near  the  edge  of  the 
water  which  lies  opposite  to  the  direction  of  the  prevailing  wind. 
These  should  be  collected  and  placed  in  a  jar  full  of  water  with 
some  Anacharis  in  the  warm  room  at  home,  where  they  will 
hatch  by  the  end  of  December  or  January,  and  the  beautiful 
young  Polyzoa  can  be  seen  emerging  from  their  box-shaped 
prison. 

December. 

Severe  weather  in  this  country  does  not,  as  a  rule,  set  in  in 
December,  and  the  lakes  and  ponds  are  not  usually  frozen  over 
in  the  early  part  of  the  month.  The  winter  fauna  has  now 
become  more  pronounced,  but  includes  quite  a  number  of 
Tnfusorians,  Rotifers,  and  Crustaceans.     The  following  species  of 


ROTIFERA  245 

rotifers  have  been  collected  in  December  in  lakes  and  canals  in 
and  round  London,  some  of  them  in  great  abundance :  Anurcea 
aculeata  and  cocldearis,  Asplcmchna  BrighttccUii  and  priodonta, 
Brachionus  annularis,  DiascJiiza  semiaperta,  Euchlanis  deflexa, 
Melicerta  ringens,  CEcistes  crystallinus,  Limnias  ceratophylli, 
Floscidaria  cornuta,  Synchceta  pectinata  and  tremnla,  ConocldliiH 
unicornis,  Rotifer  vulgaris  and  macrurm,  Polyarthra  platyptera, 
Notholca  scapha,  Triarthra  longiseta.  Of  Crustaceans,  Diaptomus 
castor  and  various  Cyclops  and  their  larvae  are  abundant,  whilst 
Water-fleas  die  down.  A  minute  red  flagellate  Infusorian  often 
seems  to  form  the  chief  food  material  of  the  above  lake  fauna. 

2.  Collecting-Grounds  near  London. 

A  few  of  the  principal  collecting-grounds  for  pond-life  in  and 
near  London  may  be  mentioned.  The  nearest  and  most  convenient 
available  piece  of  water  is  the  Grand  Junction  and  Regent's  Canal, 
which  runs  from  east  to  west,  on  the  northern  side  of  London, 
from  Victoria  Park  to  Han  well,  and  is  readily  approachable 
wherever  access  can  be  gained  to  the  towing-path.  Wimbledon 
Common  a,nd  all  the  great  parks  have  a  lake,  such  as  Victoria 
Park,  Regent's  Park,  Hyde  Park,  Richmond  Park,  etc.,  which 
all  afford  good  collecting-grounds.  Smaller  ponds  are  found  in 
abundance  in  fields  and  commons  in  and  beyond  suburban 
London,  and  I  need  only  mention  a  few  such  places  :  Epping 
Forest,  Higham  Park,  Hadley  Wood,  Totteridge,  Hampstead 
Heath,  Ealing  Common,  Hampton  Court,  Putney  Common. 

3.  Methods  of  Collecting,  Preliminary  Examination,  and 

Keeping. 

The  fascinating  study,  under  the  microscope,  of  the  living 
microscopic  objects  found  in  ponds,  canals,  and  lakes,  collectively 
known  as  '  pond-life,'  requires,  first  of  all,  that  you  should  catch 
your  game.  The  object  of  this  note,  therefore,  is  to  discuss  those 
methods  of  collecting  which,  with  a  good  many  years' experience, 
have  proved  to  me  the  most  practical,  efficient,  and  time-saving  ; 
it  is  intended  for  the  young  naturalist  or  beginner  who  desires 
to  make  the  personal  acquaintance  of  these  minute  atoms  of  life, 
and  thereby  gain  a  better  understanding  of  all  living  things. 


246  MODERN  MICROSCOPY 

A  few  pieces  of  apparatus  are  indispensable,  and  these  are  the 
following : 

1.  A  Queketter's  collecting-stick  with  ring-net  and  bottle,  and 
cutting-hook. 

2.  A  flat  bottle. 

3.  A  pocket  magnifier. 

4.  A  hand-bag  with  sundry  wide-mouthed  bottles. 

The  Collecting-Stick  can  be  obtained  from  most  opticians. 
It  is  a  hollow  walking-stick  with  an  inner  rod  to  increase  its 
length  when  required,  and  provided  with  a  screw  at  the  end  for 
the  attachment  of  either  ring-net,  dipping-bottle,  or  hook. 

The  ring  is  a  stout  brass  hoop,  about  6  inches  in  diameter. 
The  net,  which  is  sewn  on  to  the  ring,  is  made   cone-shaped, 


fi  ,B 


c 

Fig.  81. 

A,  C  =  9  in.  ;  angle  at  C  =  140°. 

about  6j  inches  long,  and  at  its  apex  is  tied  a  small  rimmed 
tube-bottle  of  clear  glass,  about  3  inches  long  by  1  inch  wide. 
The  material  of  the  net  should  be  either  fine  muslin,  known  as 
*  soft  mull,'  with  meshes  fine  enough  to  prevent  the  Infusoria 
and  Rotifera  going  through,  and  yet  allowing  the  water  to  run 
out  freely,  or  else  a  silk  material  known  as  '  Swiss  bolting  silk,' 
used  by  millers  for  sifting  the  various  grades  of  flour,  and 
obtainable  from  all  mill  furnishers  ;  No.  16  of  this  silk  material 
has  the  required  fineness. 

The  net  is  most  important,  and  some  care  should  be  taken  to 

have  it  properly  made.     Allowing  a  margin  for  the  seam  and  for 

sewing  round  the  ring,  the  shape  and  dimensions  of  the  material 

for  a  6-inch  ring  should  be  as  represented  in  Fig.  81.     This  will 

give  a  net  slightly  larger  than  is  required,  but  as  the  material 


EOTIFERA  247 

is  sure  to  shrink  a  little,  it  will  be  of  the  right  size  after  having 
been  used  once  or  twiceo 

The  cutting-hook  is  a  curved  knife  which  can  also  be  screwed 
on  to  the  collecting-stick,  and  is  intended  for  cutting  roots  or 
water  weeds  which  are  otherwise  out  of  reach. 

The  Flat  Bottle  can  be  obtained  from  opticians,  well  made, 
and  the  parts  joined  by  fusing  with  fusible  cement.  When 
first  invented  by  the  late  Mr.  T.  D.  Hardy,  it  was  made  by 
cutting  a  Li-shaped  piece  out  of  a  thick  flat  piece  of  india-rubber 
or  similar  material,  4  to  5  inches  long,  by  2  to  2J  inches  wide, 
and  f  to  |  inch  thick  ;  a  square  of  thin  plate-glass  of  same 
size,  cemented  by  means  of  Miller's  caoutchouc  cement  on  each 
side,  completed  the  bottle.  A  thick  piece  of  india-rubber  is, 
however,  so  expensive  that  it  is  cheaper  to  buy  the  finished 
article.  The  flat  bottle  is  used  for  searching  over  pond-weeds 
with  the  pocket  lens  at  the  side  of  the  pond,  or  examining  the 
water  which  has  been  collected  and  condensed  with  the  net.  In 
round  bottles  it  is  very  difficult  to  see  minute  animals  clearly, 
whilst  a  thin  flat  bottle  allows  the  whole  contents  to  be  readily 
scrutinized  with  a  pocket  lens  of  considerable  power,  and  one 
can  at  once  determine  whether  it  is  worth  while  to  take  home  a 
sample  from  that  particular  pond  for  further  examination  under 
the  microscope. 

.  The  Pocket  Magnifier  best  adapted  for  field-work  is  Zeiss's 
improved  aplanatic  lens,  magnifying  six  diameters,  which  has  a 
very  large  flat  field,  long  focus,  and  perfect  definition  all  over 
the  field. 

The  various  groups  of  plants  and  animals  commonly  desig- 
nated as  'pond-life,'  which  inhabit  fresh-water  lakes,  ponds,  and 
ditches,  consist  of  Algaa,  Desmids,  Rhizopoda,  Infusoria,  Sponges, 
Hydras,  Rotifera,  Polyzoa,  Cladocera  or  Water-fleas,  Copepods, 
Hydrachnida,  Worms,  and  Insect  larvae.  All  these  can  be 
divided  for  the  purpose  of  collecting  into  two  groups — the  free- 
swimming,  and  those  that  are  usually  attached  to  water-plants 
or  submerged  objects,  and  each  of  these  groups  must  be  captured 
in  different  ways. 

All  free-swimming  or  floating  forms,  which  collectively  are 
designated  by  the  word  'plankton,'  are  best  secured  with  the 
net.     The  net  is  passed  through  the  water  two  or  three  or  more 


248  MODERN  MICROSCOPY 

times,  and  then  held  up  ;  the  water  will  run  out  in  half  a  minute 
and  quite  at  the  last  the  condensed  animals  will  be  seen  entering 
the  little  bottle  like  a  cloud,  where  they  can  be  subjected  to  a 
preliminary  examination.  It  is  best,  however,  to  empty  the 
contents  into  the  flat  bottle,  in  which  the  examination  with  the 
pocket  lens  becomes  very  much  easier,  and  most  of  the  forms 
one  is  acquainted  with  can  be  recognized  at  a  glance.  In  this 
way  thousands  of  Algse,  Infusoria,  Rotifera,  Daphnia,  etc.,  can 
be  captured  in  a  few  minutes  if  the  pond  be  a  prolific  one. 
Having  thus  ascertained  that  the  dip  contains  some  desirable 
forms,  the  water  is  poured  into  a  large,  wide-mouthed  collecting- 
bottle,  of  which  three  to  six  should  be  carried  in  the  bag.  These 
bottles  should  be  numbered ;  for  it  is  often  advantageous  to  keep 
the  water  of  different  ponds  separate,  so  as  to  be  able  to  know  at 
home  from  which  pond  a  particular  creature  has  come.  Ponds 
vary  exceedingly  as  regards  their  contents  in  pond-life ;  a  small 
pond  may  be  very  prolific,  whilst  another,  possibly  a  larger  piece 
of  water  only  a  few  yards  off,  may  contain  hardly  anything 
worth  collecting.  By  trying  all  the  different  ponds,  small  and 
large,  within  reach  of  an  afternoon's  walk,  one  usually  succeeds 
in  obtaining  a  good  gathering  of  free-swimming  forms.  The 
net  quickly  condenses  a  large  volume  of  water,  so  that  few 
species,  even  if  present  in  small  numbers  only,  will  escape 
being  captured.  Several  other  methods  of  condensing  pond- 
water  have  been  devised,  but  the  collecting-net  with  bottle 
attached  is  so  simple  and  effective  that  we  need  not  trouble 
about  any  other  apparatus.  It  may  be  advisable  to  try  the 
larger  ponds  in  various  places,  and  both  near  the  surface  and 
also  in  deep  water,  as  some  plankton  forms  may  have  collected 
in  one  particular  corner  of  the  pond  and  be  absent  elsewhere  ; 
this  is  often  the  case  with  Volvox  globator.  The  use  of  a  boat 
on  larger  lakes  is  very  desirable  wrhen  available.  For  rotifers 
and  other  active  free-swimmers  it  is  not  desirable  to  disturb  the 
mud  at  the  bottom  of  the  pond,  but  certain  species  of  Cladocera, 
Hydrachnida,  and  insect  larvae  can  only  be  found  at  or  near  the 
bottom. 

The  group  of  attached  forms  of  pond-life  comprise  such 
Infusoria  as  Carchesium,  Epistylis,  Zoothamnium,  Stentor,  etc.; 
Hydra;  all  Polyzoa  and  Sponges.     In  searching  for  these  forms, 


ROTIFEKA  249 

a  quantity  of  pond- weeds,  or  rootlets,  are  brought  on  shore  with 
the  cutting-hook,  and  selecting  some  likely-looking,  fairly  clean 
branches,  but  not  the  newest  growth,  one  twig  after  another  is 
placed  in  the  flat  bottle  in  clean  water,  where  it  can  be  examined 
from  both  sides  with  great  ease,  both  with  the  naked  eye  and 
the  pocket  lens.  The  tree-like  Yorticella  colonies— Epistylis, 
Zoothamnium,  Carchesium  ;  the  trumpet-shaped  Stentors ;  the 
Crown  Rotifer  Stephanoceros ;  the  tubes  of  Melicerta  and  Limnias ; 
the  various  Polyzoa  ;  also  Hydra  and  Sponges,  and  many  others, 
can  at  once  be  seen  when  present,  and  in  this  way  good  branches 
can  be  selected  and  placed  in  a  separate  wide-mouthed  collect- 
ing-bottle containing  clean  pond-water.  A  little  experience  will 
soon  teach  one  which  branches  are  likely  to  prove  prolific.  As 
a  general  rule  one  may  say  that  old-looking,  but  still  sound  and 
green,  branches  are  the  best.  The  Water  Milfoil  (Myriophyllum) 
is  one  of  the  best  water-plants  to  examine  and  collect  on  account 
of  the  ease  with  which  its  leaves  can  subsequently  be  placed 
under  the  microscope.  Anacharis  is  more  troublesome,  but  it 
is  occasionally  found  covered  with  pond-life,  and  is  an  excellent 
weed  for  the  aeration  of  aquaria. 

The  rootlets  of  reeds  and  of  trees  growing  near  the  edge  of 
the  water  should  be  examined  for  Sponges  and  Polyzoa,  such 
as  Lophopus,  Plumatella,  Fredericella,  etc.  In  order  to  obtain 
some  weeds  growing  near  the  middle  of  a  pond  or  lake,  a  loaded 
three-pronged  hook,  attached  to  a  line,  may  be  used  ;  this  is 
swung  round,  and  may  be  thrown  to  a  distance  of  20  to  25  yards, 
where  it  sinks,  and  the  weeds  that  are  caught  by  the  hooks  are 
dragged  on  shore. 

By  these  various  means  a  good  collection  of  pond  organisms 
can  readily  be  made  after  a  little  practice.  Though  the  spring 
and  autumn  are  perhaps  the  best  seasons  for  collecting,  pond-life 
is  never  absent,  even  in  the  winter  under  the  ice. 

Having  thus  filled  some  bottles  with  condensed  water  from 
various  ponds,  and  placed  some  promising  branches  of  water- 
plants  in  another  bottle  filled  with  uncondensed  and  clean  pond- 
water,  the  '  bag '  is  taken  home.  It  is  a  great  mistake,  however, 
to  overstock  the  bottles  with  weeds,  as  the  plants  in  such  crowded 
bottles  may  begin  to  decompose,  killing  most  of  the  animals  in  a 
short  time. 


250  MODERN  MICROSCOPY 

On  reaching  home,  the  first  thing  to  do  is  to  empty  the  collect- 
ing-bottle into  larger  vessels  or  small  aquaria,  in  such  a  way 
that  the  captures  may  be  critically  examined,  isolated,  and,  if 
found  desirable,  placed  under  the  microscope.  By  far  the  best 
and  most  convenient  way  of  doing  this  is  to  transfer  the  contents 
of  each  bottle  into  a  small  window  aquarium,  filling  it  up  with 
tap-water.  The  weeds  and  rootlets  that  have  been  brought  home 
are  put  in  another  window  aquarium  in  clean  pond-water. 

These  small  window  aquaria,  with  flat  and  parallel  sides 
6  to  8  inches  long  by  5  to  6  inches  high,  and  only  1\  inches  wide 
inside,  are  the  best  nurseries  for  the  microscope.  The  difficulty 
of  seeing  and  capturing  small  objects  in  a  large  or  ordinary  round 
aquarium  is  very  great,  and  the  use  of  the  pocket  lens  almost 
hopeless,  whilst  in  these  flat  and  narrow  aquaria  no  object  is  out 
of  reach  of  the  lens,  and  the  whole  contents  can  be  looked  over 
without  difficulty  and  in  a  very  short  time. 

By  placing  the  tank  on  a  what-not  at  a  convenient  height 
before  a  window,  or  before  a  lamp  at  night,  most  of  the  free- 
swimming  rotifers  will  collect  against  the  glass  nearest  to  the 
light,  where  they  can  be  examined  with  the  greatest  ease  and 
picked  up  with  the  pipette  if  desired.  A  disc  of  black  cardboard 
placed  some  little  distance  behind  produces  a  very  good  dark 
ground,  against  which  the  smallest  visible  specks  stand  out  well. 

The  condensed  pond-water  is,  of  course,  frequently  so  dirty 
with  floating  particles  of  debris  that  it  is  at  first  hardly  possible 
to  see  through  it ;  but  after  standing  half  an  hour  it  will  be 
found  that  most  non-living  particles  will  have  fallen  to  the 
bottom,  and  after  several  hours  the  water  will  be  quite  clear  and 
and  every  living  creature  will  be  readily  seen. 

During  the  summer  months,  when  Daphnia  and  Cyclops  are 
abundant,  the  net  frequently  collects  these  in  such  numbers  that 
they  become  a  nuisance.  In  order  to  separate  them,  when  such 
is  the  case,  I  have  adopted  the  plan  of  passing  the  water  through 
a  small  sieve  made  of  material  with  meshes  sufficiently  wide  to 
allow  the  largest  rotifers  and  Infusoria  to  go  through,  whilst 
keeping  back  most  of  the  Cyclops  and  Water-fleas  ;  the  latter  are 
then  transferred  to  a  separate  tank  to  be  examined  by  them- 
selves. 

It  is  very  desirable  to  examine  the  collected  objects  as  soon  as 


ROTIFEEA  251 

convenient,  the  same  day  if  at  all  possible,  and  not  later  than 
the  day  after  their  capture,  as  many  organisms  soon  die  and 
disappear  under  the  crowded  and  unnatural  conditions  in  which 
they  are  kept  in  captivity.  Eotifers  can  often,  particularly  in 
cool  and  cold  weather,  be  kept  for  a  week  or  fortnight,  and  some 
species,  such  as  Melicerta,  occasionally  for  months  if  food 
material  in  the  shape  of  fresh  pond- water  can  be  provided. 
Failing  pond-water,  water  from  hay  infusions,  which  mostly  con- 
tain quantities  of  bacteria  and  minute  Infusoria,  may  be  added. 
The  various  species  of  Polyzoa  and  Sponges  can  also  be  kept 
alive  a  considerable  time  by  feeding  them  in  a  similar  way,  but 
Hydras  require  a  fare  of  Water-fleas  if  they  are  to  thrive. 

For  keeping  microscope  life  I  have  found  no  difference  between 
large  and  small  aquaria,  but  the  small  tanks  are  the  more 
manageable  ;  the  great  thing  to  be  attended  to  is  the  proper 
aeration  with  water-plants,  of  which  Anacharis,  Fontenalis,  and 
Valisneria  are,  perhaps,  the  best,  and  not  to  overstock  the  tank 
with  either  animal  or  vegetable  life.  The  water  need  not  be 
changed,  but  a  little  fresh  pond- water  should  be  added  from  time 
to  time.  Larger  animals,  such  as  small  fish,  water-beetles  and 
snails  must  be  excluded  altogether  from  small  tanks,  and  Polyzoa 
and  Sponges  must  be  kept  therein  in  very  moderate  quantity  and 
small  colonies  only. 

In  order  to  insure  success  it  is  essential  to  maintain  a  proper 
balance  between  the  animal  and  the  vegetable  life,  and  also  to 
supply  fresh  food  frequently,  for  microscopical  animals  no  more 
than  the  larger  beasts  can  live  long  without  food.  To  some 
extent,  no  doubt,  they  feed  on  each  other,  but  in  a  small  aquarium 
their  hunting-ground  is  very  limited  and  the  game  soon  becomes 
scarce.  Asplanchna  can  be  seen  under  the  microscope  to  feed  on 
Anuraea,  Brachionus,  Polyarthra,  Triarthra,  and  other  rotifers 
when  it  can  catch  them,  and  their  shells  and  remains  are  fre- 
quently found  in  Asplanchna' s  stomach. 

On  the  whole,  the  best  plan  is  to  go  out  and  collect  a  fresh 
supply  from  time  to  time,  and  as  often  as  may  be  convenient. 
I  may  mention  that  at  the  middle  of  January  I  had  many 
thousands  of  rotifers  in  a  tank  which  I  collected  two  days 
before  in  the  Grand  Junction  Canal,  near  Westbourne  Park 
Station.     The  canal  was   covered  with  blocks  of   ice,  and  the 


252 


MODERN  MICROSCOPY 


time  spent  near  the  water  did  not  exceed  ten  minutes,  during 
which  I  filled  a  large  bottle  with  water  condensed  by  means  of 
the  ring-net. 

Everyone  who  has  worked  at  pond-life  will  have  experienced 
how  awkward  it  is  to  examine  with  a  pocket  lens,  and  at  the 
same  time  attempt  to  pick  out  a  particular  animal  in  order  to 
place  it  under  the  microscope.  In  order  to  have  both  hands 
free  for  this  operation,  and  to  keep  the  lens  fixed  to  a  particular 
spot,  I  devised  some  years  ago  a  small  aquarium  microscope 
(Fig.  82),  which  is  simply  a  flat  metal  arm,  jointed  in  such  a 


Fig.  82. 


way  that  it  allows  the  lens  to  be  moved  all  over  the  surface,  but 
in  one  plane  only,  parallel  to  the  side  of  the  window  aquarium, 
whilst  the  lens  is  focussed  by  a  small  rack  and  pinion  on  the 
left.  The  whole  apparatus  is  screwed  to  a  small  wooden 
stand,  on  which  the  tank  is  placed.  The  lens  used  is  Zeiss's 
aplanatic  combination  x  6  diameters,  which  has  working  dis- 
tance enough  to  focus  right  through  the  tank,  and  sufficient 
amplification  to  enable  one  to  recognize  most  rotifers,  Infusoria, 
etc.,  and  anything  uncommon  or  new  can  at  once  be  detected 
and  secured.  Moving  objects  can  readily  be  followed  with  the 
lens,  and  pond-weeds  can  be  searched  for  anything  that  may  be 
growing  on  them,  whilst  the  lens  remains  fixed  in  any  position 
it  may  be  placed.     I  have  had  this  tank  microscope  in  constant 


ROTIFEKA 


253 


use  for  over  twelve  years,  and  can  recommend  it  as  thoroughly 
practical,  efficient,  and  time-saving. 

4.  Apparatus  for  Microscopic  Examination. 

I  propose  now  to  describe  those  methods  which  long  experience 
has  proved  to  be  the  most  practical  in  the  examination  of  living 
objects  under  the  microscope. 

Fig.  83  is  a  photograph  of  various  apparatus  used  for  this 
purpose,  consisting  of  troughs,  pipettes,  live-box,  and  compressor. 

After  capturing  a  miscellaneous  collection  of  pond-life  and 
transferring  it  to  a  window  aquarium  placed  in  front  of  a  window, 


a- 


f— 


Fig.  83. 

as  previously  explained,  it  will  be  desirable  first  of  all  to  place 
some  of  it  under  a  low  power  of  the  microscope— say  a  2-inch  or 
li-inch  objective — in  order  to  obtain  a  better  general  view  of  the 
various  animals.  The  free-swimming  forms  will  mostlv  have 
collected  on  the  light  side  of  the  aquarium,  and  can  there  be 
picked  up  quite  clean  and  in  vast  numbers,  sometimes  with  the 
pipette  e,  and  transferred  to  a  square  trough  a  or  bf  and  placed 
under  the  microscope,  where  the  contents  can  readily  be  illumin- 
ated from  below,  both  with  transmitted  light  and  under  dark 
ground.  I  prefer  to  use  dark-ground  illumination  with  low 
powers  when  searching  over  the  contents  of  a  trough,  and  when 
studying  the  shape,  mode  of  swimming,  ways  of  feeding  and 
living  of  Polyzoa,  Rotifera,  and  Infusoria.  Moreover,  the 
animals  scattered  through  the  trough  will  soon  collect  in  the 


254  MODERN  MICROSCOPY 

spot  of  light  of  the  condenser,  and  then  the  whole  field  of  view 
will  often  be  a  mass  of  moving,  dancing,  tumbling,  sparkling 
life. 

The  trough  a,  3  inches  by  1J  inches  and  T%  inch  thick,  is  the 
form  I  mostly  use ;  it  stands  upright  on  the  table,  is  reversible, 
and  can  be  handled  without  greasing  the  well  part  of  the  glass. 
The  sides  are  cemented  in  the  fire  by  means  of  a  fusible  glass 
cement,  and  thus  the  trough  is,  and  remains,  watertight.  The 
trough  b  is  also  a  useful  type,  but  it  is  not  reversible,  will  not 
stand  by  itself  on  the  table,  and,  being  cemented  with  gold  size 
or  marine  glue,  is  liable  to  leak.  The  troughs  usually  sold  are 
semicircular  in  shape,  a  very  bad  type,  because,  in  addition  to 
the  above  defects,  the  least  amount  of  tilting  on  the  stage  will 
cause  the  water  to  run  out  over  the  edge.  Thicker  troughs 
are  objectionable  because  the  sub-stage  condenser  cannot  work 
through  them,  and  the  animals  cannot  be  properly  illuminated, 
though  sometimes  such  troughs  may  be  required  by  the  size  and 
nature  of  the  object. 

A  few  words  on  pipettes  will  not  be  out  of  place  here.  The 
old-fashioned  way  of  using  the  finger  on  a  straight  or  curved 
glass  tube  to  capture  pond-life  is  so  unsatisfactory  that  I  have 
been  driven  to  invent  new  pipettes  for  more  precise  and  exact 
work.  Fig.  83,  c,  d,  and  e,  represent  the  pipettes  in  constant 
use  ;  e  is  a  glass  tube  about  yV  inch  in  diameter  and  8  inches 
long,  which  tapers  from  the  middle  to  a  point  more  or  less  fine, 
according  to  the  size  of  the  animals  one  wishes  to  capture. 
Over  the  wide  end  is  placed  an  india-rubber  teat,  by  means  of 
which  any  single  specimen,  or  scores  of  animals,  can  be  sucked 
up  with  the  least  quantity  of  water ;  d  is  another  type  of 
pipette,  having  a  still  finer  action  ;  it  is  6  inches  long,  funnel- 
shaped  at  one  end,  and  tapering  gradually  from  the  funnel  to  a 
fine  point ;  the  funnel  is  f  inch  wide,  and  covered  with  an  india- 
rubber  membrane  ;  c  is  a  similar,  but  smaller  and  finer  pipette, 
3J  to  4  inches  long,  for  picking  up  small  rotifers  in  a  fraction 
of  a  drop  of  water  under  the  dissecting  microscope.  The 
slightest  touch  on  the  membrane  is  sufficient  to  expel  or  bring 
in  the  water,  so  that  one  has  complete  control  over  the  amount  of 
water  that  is  taken  up,  and  there  is  much  less  risk  of  losing  the 
animal  one  wishes  to  transfer  to  the  compressor. 


ROTIFEKA  255 

The  old-fashioned  live-box  with  raised  tablet,  still  largely  sold 
with  microscopes,  is  quite  useless  for  pond-life,  for  the  simple 
reason  that  the  objects  cannot  be  properly  illuminated  with  the 
sub-stage  condenser.  This  consideration  led  me  long  ago  to 
design  the  live-box/,  in  which  the  glass  tablet  is  fixed  flush  with 
the  brass  plate,  and  is  of  small  size,  thus  leaving  a  wide  ring  all 
round.  This  arrangement  allows  all  objects  on  the  tablet  to  be 
perfectly  illuminated  from  below  by  the  achromatic  condenser, 
both  with  transmitted  light  and  under  dark  ground,  and  at  the 
same  time  they  can  be  reached  and  followed  from  above  with 
both  low  and  high  powers  and  oil  immersion  lenses,  to  the  very 
edge  of  the  tablet,  and  wherever  they  may  wander.  For  more 
exact  work,  when  it  is  desired  to  hold  a  single  rotifer  between 
the  two  glasses,  and  prevent  its  wandering  about,  I  have  devised 
the  compressor  g,  in  which  the  pressure  and  the  thickness  of  film 
of  water  can  be  accurately  regulated  by  a  screw  acting  against  a 
spiral  spring.  At  the  same  time,  water,  or  reagents,  can  be 
added,  if  desired,  without  raising  the  cover.  When  properly  and 
well  made,  this  compressor  works  exceedingly  well,  and  I  have 
had  it  in  constant  use  for  years,  but  some  makers,  unfortunately, 
have  introduced  variations  and  so-called  '  improvements '  which 
just  take  away  some  of  the  essential  and  useful  points.  The 
semicircular  thin  cover-glass  must  be  cemented  to  the  under  side 
of  the  brass  ring  with  a  little  gold  size,  so  as  to  be  quite  firm 
and  rigid,  otherwise  its  action  becomes  uncertain,  and  very  small 
objects  cannot  be  held  fast,  or  else  are  suddenly  crushed. 

Some  more  simple  apparatus  and  devices  may  be  mentioned 
for  cases  where  no  live-box  or  compressor  is  at  hand.  An 
excavated  glass  slide  makes  a  fair  live-cage ;  a  drop  of  water  con- 
taining the  animals  is  placed  in  the  cavity  so  as  to  just  fill  it,  and 
no  more ;  another  drop  of  clean  water  is  placed  by  the  side  of  the 
cavity,  and  a  clean  thin  cover-glass  is  lowered  on  to  that  second 
drop  ;  then,  by  means  of  a  needle,  the  cover  is  slowly  pushed 
across  the  cavity,  which  can  thus  be  covered  without  enclosing 
an  air-bubble  ;  the  superfluous  water  is  taken  up  by  blotting- 
paper,  the  cover-glass  being  held  in  position  by  capillary  attrac- 
tion. This  forms  a  good  slide  for  low  and  medium  powers,  but 
not  for  high  powers.  Another  good  temporary  slide  can  be  made 
by  placing  three  small  fragments  of  Xo.  1  thin  cover-glass  near 


256  MODERN  MICROSCOPY 

the  middle  of  a  glass  slip  in  form  of  a  triangle  ;  the  drop  of 
water  containing  the  animals  is  placed  in  the  centre,  and  a  clean 
thin  cover-glass  is  lowered  on  to  the  drop  so  as  to  rest  on  the 
three  glass  fragments,  which  prevent  the  animals  getting  crushed. 
If  there  be  too  much  water  it  can  be  removed  with  blotting-paper. 
Low  and  high  powers  can  be  used  on  this  slide  as  far  as  the 
movements  of  the  animals  will  permit,  but  not  oil  immersion 
lenses. 

Having  thus  mentioned  some  essential  and  necessary  apparatus, 
I  will  close  with  a  few  remarks  on  the  examination  of  living  pond- 
life.  The  free-swimming  organisms,  including  such  forms  as 
Volvox  globator,  collect  on  the  light  side  of  the  window  aquarium, 
and  can  there  be  picked  up  in  small  or  large  numbers,  and  quite 
clean,  with  the  large  pipette,  and  placed  in  the  trough ;  or  any 
particular  species  can  be  selected  with  the  aid  of  the  tank  micro- 
scope, and  taken  up  with  the  smaller  pipette,  and  transferred  to 
the  live-box  or  compressor  in  a  single  drop  of  clean  water,  both 
hands  being  free  for  this  operation. 

The  fixed  forms,  such  as  Polyzoa,  Stephanoceros,  Melicerfca, 
Floscules,  etc.,  amongst  Eotifera,  and  Stentor,  Carchesium, 
Zoothamnium,  etc. ,  amongst  Infusoria,  require  a  little  manage- 
ment. If  simply  placed  in  a  trough,  these  are  often  obscured  or 
incapable  of  being  properly  illuminated  by  being  too  crowded,  or 
by  part  of  the  weed  over-  or  under-lying  the  objects,  and  also  by 
floating  particles  in  the  water.  The  best  result  is  obtained  by 
trimming — that  is,  by  cutting  off  a  very  small  piece  of  weed  or 
leaf  on  which  the  animal  is  attached — in  a  watch-glass  under  the 
dissecting  microscope,  if  necessary — and  then  transferring  it  with 
the  pipette  to  the  compressor  into  a  drop  of  clean  water ;  it  can 
then  be  arranged  with  a  needle  or  bristle  as  may  be  desired,  and 
after  lowering  the  cover-glass,  fixed  and  held  fast,  at  the  same 
time  giving  the  animal  perfect  freedom  to  expand.  In  this 
position  the  animals  can  be  reached  with  the  achromatic 
condenser  from  below  for  transmitted  light  and  dark-ground 
illumination,  and  also  with  low  and  high  powers,  and  even  oil 
immersion  objectives  from  above. 

For  pond-life  work  the  Wenham  binocular  is  decidedly  to  be 
preferred  to  the  monocular  microscope.  It  is  less  tiring  to  the 
eyes  to  look  with  both  eyes  and  without  strain,  and  the  stereo- 


ROTIFERA  257 

scopic  image  gives  a  very  much  better  idea  of  the  true  shape  of 
the  animals,  though  the  images  are  not  quite  so  sharp  as  with 
the  monocular  tube ;  but  this  binocular  form  can  immediately 
be  changed  into  a  monocular  for  high-power  work,  or  whenever 
desired,  by  pushing  the  small  prism  out  of  the  way. 

The  binocular  is  to  be  used  only  with  the  low  powers  up  to 
the  f-inch  objective ;  with  higher  powers  the  stereoscopic  effect 
is  lost,  because  the  depth  of  focus,  or  the  plane  of  distinct  vision, 
is  then  exceedingly  small,  and  becomes  more  and  more  a  mere 
optical  section  of  the  object. 

A  mechanical  stage  is  hardly  necessary ;  for  ordinary  work  a 
well-made  sliding  stage  or  bar  is  preferable,  and  should  be  pro- 
vided. Stage-clips,  of  which  opticians  are  so  fond,  are  abomina- 
tions, and  should  be  consigned  to  the  dust-bin. 

Of  illuminating  apparatus,  the  Abbe  form  of  sub-stage  con- 
denser, achromatic  if  possible,  is  the  only  one  that  is  really 
useful  for  all  powers,  and  that  need  be  considered  both  for 
transmitted  light  and  for  dark- ground  illumination.  It  should 
be  provided  with  an  iris  diaphragm  and  an  arm  carrying  a 
central  stop ;  it  completely  replaces  all  the  older  sub-stage 
apparatus — condenser,  spot  lens,  paraboloid,  etc.  The  bull's- 
eye  stand  condenser,  however,  is  necessary  to  render  parallel  the 
rays  of  the  lamp-flame,  but  it  should  be  mounted  on  the  lamp, 
and  move  about  with  the  lamp,  and  so  as  to  project  an  enlarged 
image  of  the  edge  of  the  flame  on  to  the  flat  mirror  of  the  micro- 
scope for  dark-ground  illumination  with  low  powers. 

All  apparatus  used  in  the  examination  of  pond-life — troughs, 
live-boxes,  compressors,  and  pipettes — should  always  be  carefully 
cleaned  and  dried  immediately  after  use,  and  in  no  case  should 
the  water  be  allowed  to  evaporate  in  them.  Much  trouble  will 
be  saved  by  the  observation  of  this  rule,  and  the  apparatus  will 
always  be  ready  for  use. 


5.  Preserving  and  Mounting. 

There  are  few  observers  of  pond-life  who  have  not  felt  a  keen 
desire  to  preserve  and  keep  these  small  highly  organized  sparks 
of  life  instead  of  letting  them  die  and  disappear  in  a  few 
days.     For   a   close   study  of   this   group,  well-preserved   type 

17 


258  MODEEN  MICROSCOPY 

specimens  are  of  the  greatest  possible  assistance  and  importance, 
and  if  such  had  existed  formerly  much  confusion  and  inexacti- 
tude in  their  description  and  classification  would  have  been 
avoided,  particularly  in  the  giving  of  three  or  four  different 
names  to  the  same  species,  which  causes  so  much  trouble  to  the 
student. 

The  total  absence  of  type  specimens  of  rotifers  to  refer  to 
when  required,  originally  led  to  an  attempt  on  the  part  of 
Mr.  C.  F.  Eousselet  to  produce  them,  and  it  is  now  over  ten 
years  since  the  first  successful  experiments  at  preserving  them 
in  a  fully  extended  and  natural  state  were  made.  His  method, 
although  so  simple  now,  took  fully  three  years  to  work  out  until 
the  right  and  most  suitable  narcotic,  fixing  agent,  anc  preserving 
fluid  were  found.  By  the  use  of  suitable  fixing  agents  not  only 
the  external  shape  of  rotifers  can  be  preserved,  but  also  all  the 
internal  structure,  to  the  minutest  anatomical  details,  such  as 
the  striated  muscle  fibres,  nerve  threads,  vibratile  tags  or  flame 
cells,  sense  hairs,  cilia,  etc.,  and  frequently  important  details 
can  be  more  readily  observed  than  in  the  living  animal. 

Narcotizing. — As  is  well  known,  no  killing  agent  is  sufficiently 
rapid  to  prevent  the  complete  retraction  of  rotifers,  and  few 
other  animals  can  contract  into  such  a  shapeless  mass  when  we 
attempt  to  kill  them  by  ordinary  means,  such  as  poisons,  alcohol, 
heat,  etc.  It  is,  therefore,  necessary  to  use  first  a  suitable 
narcotic,  which  has  been  discovered  in  hydrochlorate  of  cocaine. 
As  a  result  of  many  trials,  the  best  solution  for  most  rotifers  has 
been  found  to  be  the  following  mixture : 

2  per  cent,  solution  of  hydrochlorate  of  cocaine,  3  parts  ; 
alcohol  (or  methylated  spirit),  1  part ;  water,  6  parts. 

Another  narcotic  which  is  also  very  suitable  for  rotifers  is  a 
1  per  cent,  watery  solution  of  hydrochloride  of  eucaine,  recom- 
mended by  Mr.  G.  T.  Harris,  for  Infusoria  and  other  animals. 
These  narcotics,  even  so  dilute,  are  not  to  be  used  pure,  as  they 
would  cause  the  rotifers  to  contract  at  once  and  not  expand 
again.  The  principle  to  be  followed  throughout  is  to  use  the 
narcotic  so  weak  that  the  animals  will  not  mind  it  at  first,  but 
continue  to  expand  or  swim  about  freely.  After  a  short  time 
its  effect  will  make  itself  felt  on  their  nervous  system,  and 
then  some  more  of  the  narcotic  may  be  added,  until  complete 


EOTIFERA.  259 

narcotization  is  produced,  or  until  the  animals  can  be  killed 
without  contactings. 

But  before  the  operation  of  narcotizing  is  begun,  it  is  very 
necessary  to  isolate  the  rotifers  in  perfectly  clean  water.  The 
best  way  is  to  pick  them  up  under  a  dissecting  microscope  by 
means  of  a  very  finely  drawn-out  pipette,  having  a  funnel-shaped 
enlargement  at  the  other  end,  which  is  covered  with  an  elastic 
membrane.  This  pipette  forms  a  most  delicate  siphon,  by 
means  of  which  any  selected  rotifer  can  readily  be  taken  up 
with  the  least  quantity  of  water,  and  transferred  to  another 
trough  or  watch-glass  full  of  clean  water.  This  preliminary 
precaution  is  necessary,  because  particles  of  dirt  in  the  water 
readily  attach  themselves  to  the  cilia  of  dead  rotifers,  render- 
ing them  unsightly  under  the  microscope.  Another  advisable 
precaution  is  to  separate  the  different  species,  because  most 
species  require  a  slightly  different  treatment,  and  because 
the  small  species  too  readily  adhere  to  the  cilia  of  the  large 
species. 

Having  then  isolated  a  number  of  free-swimming  rotifers  in  a 
watch-glass  half  full  of  perfectly  clean  water,  one  drop  of  one  of 
the  above  narcotics  is  added  and  well  mixed.  After  five  or  ten 
minutes,  if  the  animals  continue  to  swim  about  freely,  another 
drop  is  added,  and  so  on  until  the  effect  of  the  narcotic  becomes 
visible,  and  until  the  motion  of  the  cilia  or  the  movements  of 
the  animals  slacken  or  almost  cease,  when  they  are  ready  for 
killing.  The  effect  of  the  narcotic  varies  very  much  with  dif- 
ferent species ;  some  are  most  sensitive  to  it,  whilst  others  can 
stand  a  considerable  quantity  for  a  long  time. 

Killing  and  Fixing". — Some  practice  and  patience  are  cer- 
tainly required  to  find  out  the  right  time  to  kill  the  different 
species  ;  no  general  rule  can  be  given,  as  the  time  may  vary 
from  fifteen  minutes  to  several  hours.  It  is  very  essential, 
however,  that  the  rotifers  be  still  living  when  the  killing  fluid 
is  added  to  prevent  post-mortem  changes  in  the  tissues,  which 
begin  at  once  on  the  death  of  the  animals. 

For  killing  and  fixing  several  fluids  are  suitable — namely, 
i  per  cent,  osmic  acid,  or  Flemming's  chromo-aceto-osmic  fluid, 
or  Hermann's  platino-aceto-osmic  mixture.  On  the  whole,  I 
now  prefer  the  last-named,  which  gives  a  finer  fixation  of  the 


260  MODERN  MICROSCOPY 

cellular  elements  of  the  tissues  and  does  not  stain  them  so 
much.  It  may  be  explained  that  the  term  '  fixing '  implies 
rapid  killing  and  at  the  same  time  hardening  of  the  tissues  to 
such  an  extent  as  to  render  them  unalterable  by  washing  and 
subsequent  treatment  with  preserving  fluids.  Proper  fixation 
is  very  essential,  as  no  good  preservation  can  be  obtained 
without  it. 

When  the  rotifers  are  narcotized  and  ready  for  killing,  a 
single  drop  of  one  of  the  above  fixatives  is  added,  and  mixed 
with  the  water  in  the  watch-glass.  A  few  minutes  is  sufficient 
for  fixing  small  creatures  like  these,  and  then  they  must  be 
removed  again  by  means  of  the  pipette  to  several  changes  of 
clean  water  to  get  rid  of  the  acid,  otherwise  they  will  become 
more  or  less  blackened.  When  dealing  with  marine  rotifers, 
sea-water  must  be  used  for  washing  out,  for  the  difference  in 
density  between  fresh  and  sea  water  is  sufficient  to  cause  swell- 
ing by  osmosis,  and  the  consequent  spoiling  of  the  specimen. 
After  thoroughly  washing,  the  rotifers  are  transferred  to  a 
preserving  fluid,  the  density  of  which  does  not  materially  differ 
from  that  of  water.  The  best  preserving  fluid  found  so  far  is 
a  2  J  per  cent,  solution  of  formalin,  which  is  made  by  mixing 
2J  c.c.  of  the  commercial  40  per  cent,  formaldehyde  with  37J  c.c. 
of  water,  and  then  filtering, 

The  above  are  general  directions  according  to  which  the  great 
majority  of  rotifers  can  be  preserved.  When  under  the  narcotic, 
the  animals  must  be  watched  until  it  is  seen  that  they  can  swim 
but  feebly,  when,  as  a  rule,  they  will  be  ready  for  killing.  If 
they  contract  and  do  not  expand  again,  it  is  a  proof  that  the 
narcotic  used  is  too  strong,  and  it  must  be  further  diluted.  The 
whole  method  undoubtedly  requires  great  care,  and  is  a  delicate 
operation,  which  must  be  performed  under  some  kind  of  dissect- 
ing microscope,  but  by  following  the  directions  here  given,  and 
with  some  perseverance,  anyone  can  learn  to  prepare  a  large 
number  of  species  of  rotifers.  I  would  advise  that  a  beginning 
should  be  made  with  some  such  forms  as  Brachionus,  Anursea, 
Synchseta,  Asplanchna,  Hydatina,  Triarthra,  and  Polyarthra, 
which  are  easy,  and,  moreover,  occur,  and  can,  as  a  rule,  be 
collected  in  large  numbers.  A  few  genera,  however,  are 
exceptionally   difficult.      These   are    Stephanoceros,    Floscules, 


ROTIFERA  261 

Philodina,  Rotifera,  and  Adineta,  and  it  will  be  better  to  leave 
these  until  considerable  experience  in  dealing  with  the  others 
has  been  acquired. 

It  will  have  been  noticed  that  the  rotifers  must  always  remain 
submerged  in  a  watery  fluid,  and  be  transferred  in  a  drop  by 
means  of  the  pipette.  Fluids  of  lesser  density  than  water,  such 
as  alcohol,  as  well  as  fluids  of  greater  density,  such  as  glycerine, 
are  unsuitable  because  they  set  up  strong  diffusion-currents  by 
osmosis,  which  cause  the  animals  either  to  swell  or  to  shrivel 
up  completely. 

Some  species  of  rotifers,  such  as  Triarthra,  Polyarthra, 
Pedalion,  Mastigocerca,  etc.,  have  an  outer  surface  which  is 
strongly  water-repellent,  and  when  these  come  in  contact  with 
the  surface  film  of  the  fluid  even  for  an  instant  it  is  most 
difficult  to  submerge  them  again,  and,  as  a  rule,  they  are  lost 
and  spoiled. 

Having  then  successfully  narcotized,  killed,  and  fixed  the 
rotifers  fully  extended,  and  finally  transferred  them  into  2^  per 
cent,  formalin,  the  animals  may  be  kept  in  little  bottles,  or 
mounted  in  the  same  fluid  on  micro-slides,  either  in  excavated 
cells  or  shallow  cement  cells. 

Mounting". — To  mount  on  a  slide,  place  a  drop  of  the  formalin 
solution  in  the  cell,  then  transfer  the  prepared  rotifers  into  this 
drop  with  the  pipette,  and  examine  under  the  dissecting  microscope 
to  see  that  no  particle  of  foreign  matter  has  been  introduced. 
Then  place  another  drop  of  the  fluid  on  the  slide  by  the  side  of  the 
cell,  lower  the  cleaned  cover-glass  on  that  drop,  and  push  the 
cover  cautiously  and  gradually  over  the  cavity.  The  super- 
abundant fluid  is  removed  with  blotting-paper,  and  the  slide 
closed  by  tipping  damar-gold  size  cement  all  round  the  edge  with 
a  fine  brush. 

The  permanent  closing  of  these  cells  has  been  a  matter  of  very 
considerable  difficulty.  As  the  result  of  the  experience  gained, 
it  is  recommended  that  the  cells  be  closed  first  with  a  coat  of  a 
varnish  consisting  of  two-thirds  damar  in  benzole  and  one- third 
gold  size,  then  two  coats  of  pure  shellac  dissolved  in  alcohol,  and 
finally  four  to  six  coats  of  pure  gold  size.  Each  layer  of  cement 
must  be  allowed  to  dry  thoroughly  well ;  three  days  for  each 
layer  is  not  too  long. 


262  MODEKN  MICROSCOPY 

By  the  method  described  above,  Mr.  Rousselet  has  in  the  course 
of  the  last  ten  years  made  a  collection  of  over  500  slides  contain- 
ing nearly  300  different  species  of  rotifers,  probably  the  only 
collection  of  the  kind  in  existence,  which  is  of  the  greatest  use 
for  the  identification  of  species  and  for  the  general  study  of  this 
interesting  class. 

Entomostraca  should  be  narcotized  with  the  same  solution  as 
used  by  Mr.  Rousselet  for  Rotifera,  then  killed  with  a  J  per  cent, 
solution  of  osmic  acid,  and  mounted  in  a  2 J  per  cent,  solution  of 
formalin. 


CHAPTER    XXIII 

THE  COLLECTION,  EXAMINATION,  AND  PRESERVA- 
TION  OF  MITES  FOUND  IN  FRESH  WATER 

By  C.  D.  SOAR,  F.L.S.,  F.R.M.S. 

Anyone  with  a  love  for  natural  history  wishing  for  a  hobby 
for  his  spare  time,  would  find  the  study  of  fresh-water  mites 
(Hydrachnidse)  an  extremely  interesting  one.  For  variety  and 
beauty  in  colour,  and  for  differences  in  form  and  structure,  they 
are  not  to  be  surpassed  by  any  other  organisms  found  in  fresh 
water.  Wherever  there  is  a  pond,  ditch,  or  stream,  the  collector 
is  nearly  sure  of  being  rewarded  for  his  search  by  finding  one  or 
more  species  of  these  interesting  creatures.  They  are  easily 
caught,  and  can  be  seen  with  the  naked  eye  ;  they  are,  however, 
very  seldom  recognized  without  the  aid  of  the  microscope.  They 
can  be  kept  alive  for  a  considerable  period  at  home,  and  are 
easily  preserved  when  killed. 

At  present  the  life-history  of  these  little  creatures  is  so  imper- 
fectly known  that  there  is  wide  scope  for  an  observant  naturalist. 
Although  the  life-histories  of  some  species  have  been  fairly 
investigated,  the  number  of  such  is  very  limited  compared  with 
the  species  known,  and  the  variety  of  species  which  have  been 
recorded  in  Great  Britain  are  behind  the  recorded  collections  of 
Germany  and  elsewhere. 

These  creatures  are  caught  in  three  distinct  stages — the  larval, 
the  nymph,  and  the  imago.  In  the  larval  stage  they  are  very 
small,  and  only  have  six  legs.  When  they  first  emerge  from  the 
egg  they  are  free-swimming,  but  they  soon  become  attached  as 
parasites  to  some  other  form  of  pond-life.     They  will  often  be 

Note. — The  figures  in  this  chapter  are  from  the  Journal  of  the  Quekett 

Microscopical  Club,  by  permission. 

263 


264  MODERN  MICROSCOPY 

found  hanging  like  small  red  pear-shaped  appendages  on  a  great 
number  of  aquatic  insects.  The  six  legs  they  started  life  with 
disappear  after  they  have  become  firmly  attached  by  their  mouth- 
organs  to  their  host,  and  they  spend  the  remainder  of  this  period 
of  their  existence  without  any. 

This  stage  is  succeeded  by  the  nymph  ;  the  little  creatures  are 
then  much  larger  and  have  eight  legs.  During  this  term  of  their 
existence  they  are  free-swimming,  and  can  be  caught  in  the  net 
in  numbers,  but  it  is  impossible  to  distinguish  the  sexes. 

In  the  last  stage — the  adult  or  imago — all  the  structure  and 
form  are  present,  but  many  may  be  taken  that  are  not  fully 
developed.     In  the  majority  of  species,  the  male  can  be  dis- 


Fig.  84. — Larva  of  Piona  Longipalpis.     (Kren.) 

tinguished  from  the  female  and  the  specific  differences  recog- 
nized ;  but  there  are  some  in  which  the  sexes  are  so  much  alike 
that  it  is  almost  impossible  to  tell  one  from  the  other.  In 
others,  again,  the  sexes  are  so  different — as,  for  instance,  in  the 
Arrhenuri — that  one  would  be  disinclined  to  think  they  could 
be  of  the  same  species. 

The  three  figures  are  intended  to  convey  to  the  beginner  the 
three  stages  mentioned.  Fig.  84  is  the  larva  of  Piona  longi- 
palpis.  Fig.  85  the  nymph  and  adult  of  Hydrachna  globosa 
(Geer),  showing  the  ventral  surface  and  the  epimeral  plates  to 
which  the  eight  legs  are  attached.  Fig.  86  is  the  larva  of  an 
Hydrachnid,  parasitic  on  Dytiscns  marginalia  and  Nepa  cinerea. 

There  is  another  point  in  the  adult  stage  to  which  it  will  be 
well  to  draw  attention.  When  the  mite  has  first  made  its 
appearance  from  the  inert  period  it  spends  between  the  nymph 


MITES  FOUND  IN  FRESH  WATER  265 

and  adult  stage,  the  hard  and  chitinous  parts  appear  to  be 
nearly  fully  developed,  but  the  soft  parts  are  not  so.  The  body 
often  appears  very  small,  while  the  palpi,  legs,  and  epimera,  etc., 
are  very  large  in  proportion  ;  it  is  also  very  poor  in  colour.  It 
would  be  well  to  ascertain  that  the  mites  are  quite  developed 
before  making  drawings  and  taking  measurements.  In  my 
ignorance,  when  I  first  began  the  study  of  water-mites,  I  had 
to  discard  a  number  of  drawings  I  had  made  of  different  speci- 


V 

■ 


\ 


Fig.  85. — Larva,  Nymph,  and  Adult  Hydkachna  Globosa.     (Geek.) 

mens  because  they  afterwards  proved  to  be  only  different  stages 
of  growth  of  the  same  species  of  mite. 

For  collecting  mites  there  is  no  better  apparatus  than  the 
usual  collecting-stick  used  by  pond-hunters,  having  a  metal 
ring  attachable  at  its  end  which  carries  a  cone-shaped  net  made 
of  silk  or  muslin,  with  a  glass  tube  at  the  bottom.  The 
advantage  of  the  tube  is  that  the  contents  can  be  examined 
with  an  ordinary  pocket  lens  at  any  moment  to  ascertain  if 
anything  has  been  secured  worth  preserving. 

It  is  advisable  to  carry  as  many  bottles  as  the  number  of  ponds 
that  are  likely  to  be  visited ;  careful  record  should  be  kept  of  the 


266 


MODERN  MICROSCOPY 


exact  locality  where  each  mite  is  found,  with  the  date  of  capture, 
and  this  cannot  be  done  if  all  the  specimens  are  carried  home  in 
one  bottle. 

The  most  convenient  way  of  carrying  collecting-bottles  is  by 


;  - 

M 


<*. 


X 

V 

J. 


\ 


Fig.  86. — Showing  Larvae  of  ax  Hydrachxid  Parasite  on  Dytiscts 

Marginalis  and  Nepa  Cinerea, 

h,  Dytiscus  marginalis,  showing  parasites  on  ventral  surface  and  leg ;    i,   dorsal 
surface  of  Nepa  cinerea  ;  "",  ventral  surface  of  ditto  ;  k}  larval  parasite  detached. 

sewing  two  strips  of  thick  cloth  together  with  loops  of  the 
required  size  in  the  same  manner  as  a  cartridge  bandolier. 
Such  a  device  can  be  rolled  and  stood  at  the  bottom  of  a  bag, 
and  obviates  the  chance  of  the  bottles  breaking  by  contact. 


MITES  FOUND  IN  FEESH  WATER  267 

During  the  summer  months  it  will  generally  be  found  that 
the  most  successful  captures  are  made  near  the  edges  and  in 
shallow  parts  of  ponds ;  in  the  winter-time  the  mites  get  into 
deeper  water.  Some  mites  are  to  be  found  only  on  the  mud  at 
the  bottom  of  ponds,  others  on  the  leaves  and  stems  of  water- 
plants.  In  collecting,  therefore,  it  is  necessary  to  let  the  edge 
of  the  net  just  skim  over  the  surface  of  the  mud  and  sand,  and 
up  and  down  the  stalks  and  stems  of  likely  plants. 

The  under  surfaces  of  leaves  should  also  be  scraped  with  the 
edge  of  the  net.  Anacharis  is  a  very  favourite  plant  of  water- 
mites,  and  wherever  this  is  found  it  is  almost  certain  that  mites 
will  be  secured. 

In  addition  to  the  free-swimming  mites,  there  are  a  large 
number  of  parasitic  forms,  and  it  is  as  well  to  examine  all  forms 
of  insect-life  before  discarding  material.  Fresh-water  mussels, 
in  particular,  also  the  large  water-snails  and  water-beetles,  are 
specially  to  be  recommended  for  examination,  and  once  more  let 
me  emphasize  that  if  anything  is  found,  notes  should  be  made 
of  dates,  places,  and  general  details  of  the  captures. 

On  reaching  home  the  contents  of  the  bottle  should  be  emptied 
into  a  porcelain  dish  such  as  photographers  use,  when  it  will  be 
noticed  that  the  mites  generally  swim  in  the  corners  or  along  the 
sides,  and  can  then  be  removed  with  a  pipette  to  a  large  tube 
filled  with  clean  water  in  which  some  Anacharis  is  placed.  This 
latter  will  keep  the  water  clean  and  fresh  for  a  considerable  time. 

Experience  will  dictate  which  species  can  safely  be  kept 
together,  a  matter  in  which  some  discrimination  is  required, 
because  some  varieties  prey  on  others — such,  for  instance,  as 
Limnesia  on  Eulais. 

Undoubtedly  the  best  plan  is  to  proceed  with  the  examination 
at  once,  because  a  great  part  of  the  brilliancy  of  colouring  is 
lost  in  a  short  time,  and  the  mites  are  much  more  lively  when 
freshly  caught  than  subsequently.  I  have,  however,  kept  mites 
alive  in  a  tube  4  inches  by  1  inch,  by  adding  fresh  water  to 
replace  that  evaporated,  for  a  period  of  twelve  months. 

The  best  method  of  examination  is  to  place  the  mite  on  a 
3-inch  by  1-inch  glass  slip,  turning  the  specimen  on  the  ventral 
or  dorsal  side  as  may  be  required,  and  having  every  part  extended. 
A  cover-glass  is  then  laid  over  the  specimen,  and  sufficient  clean 


268  MODEEN  MICEOSCOPY 

water  is  allowed  to  flow  between  the  cover-glass  and  the  slip  to 
fill  the  intervening  space.  The  specimen  may  move  its  limbs 
and  palpi  for  a  short  time,  but  soon  becomes  quite  passive,  the 
weight  of  the  cover-glass  being  sufficient  to  retain  the  body  of 
the  mite  in  position.  The  slip  is  then  laid  on  a  piece  of  white 
card  on  the  stage  of  the  microscope,  and  illuminated  by  reflected 
light ;  a  lj-inch  objective  will  usually  be  found  the  most  suitable. 

The  advantage  of  this  arrangement  is  that  the  specimen  can 
be  reversed,  and  both  sides  examined,  and  by  having  an  aperture 
in  the  cardboard,  a  further  examination  may  be  made  by  trans- 
mitted light.  In  this  latter  condition  the  hairs  and  claws  can 
be  seen  very  distinctly,  particularly  if  the  light  be  thrown  a  little 
obliquely.  After  examination  the  specimens  can  be  returned  to 
the  tube,  and  are  usually  none  the  worse. 

To  preserve  the  specimens  they  should  be  placed  in  the 
following  solution  : 

10  parts  glycerine,  10  parts  distilled  water,  3  parts  citric  acid, 
3  parts  pure  spirit. 

They  can  be  placed  in  the  solution  alive,  and  although  at  first 
the  limbs  will  be  contracted,  they  subsequently  retract.  It  also 
preserves  the  colours  of  hard-skinned  mites  fairly  well. 

If  at  any  time  it  is  desired  to  make  a  mounted  preparation  of 
any  mites  preserved  in  this  way,  they  can  be  transferred  to  cells 
containing  the  same  solution.  If  required  for  balsam  mounts, 
the  glycerine  can  be  removed  by  repeated  soaking  in  absolute 
alcohol,  subsequently  passing  them  through  clove  oil. 

It  will  be  found  that  balsam-mounted  specimens  will  have  a 
tendency  to  vaporize ;  this  can  be  obviated  by  making  a  small 
hole  in  the  body  of  the  mite  in  a  position  which  is  of  no  con- 
sequence, and  thus  allowing  the  balsam  to  penetrate.  I  think 
the  soft-skinned  mites  mount  best  in  glycerine  solution ;  I  do 
not  mount  in  this  medium  myself,  but  have  some  beautiful 
preparations  by  Mr.  Taverner,  in  which  the  construction  is 
shown  to  the  best  advantage.  They  have  been  in  my  possession 
for  some  time,  and  show  no  signs  of  deterioration. 

Should  any  readers  take  up  the  study  of  these  beautiful 
creatures,  dates  of  collecting,  localities  where  discovered,  and 
particulars  of  anything  they  may  have  observed  new  in  the  life- 
history,  particularly  varieties  of  colouring,  should  be  carefully 


MITES  FOUND  IN  FEESH  WATER  269 

kept,  together  with,  if  possible,  drawings.  There  is  one  mite, 
Piona  rufa  Koch,  which  has  been  found  in  England  in  three 
distinct,  bright,  and  beautiful  colours — viz.,  red,  green,  and 
brown. 

The  two  best  textbooks  on  fresh-water  mites  are  in  German — 
1  Deutschlands  Hydrachniden,'  by  Dr.  R.  Piersig,  rather  an  ex- 
pensive work,  with  about  500  pages  of  letterpress  and  51  plates ; 
and  a  number  of  '  Das  Tierreich  '  on  the  Hydrachnidae,  by 
Dr.  R.  Piersig,  Berlin,  1901.  This  contains  the  account  of 
every  known  species  up  to  date  of  publication. 


CHAPTER  XXIV 

COLLECTING  AND  PEEPAEING   FOEAMINIFEEA  * 
By  ARTHUR  EARLAND,  F.K.M.S. 

The  foraminifera,  in  spite  of  their  beauty,  the  important  part 
which  they  have  played  in  the  building  up  of  our  earth,  and 
the  many  interesting  features  of  their  life-history,  have  not  met 
with  so  much  favour  among  microscopists  as  many  groups  of  far 
less  importance.  This  comparative  neglect  is  largely  due  to 
mistaken  ideas  as  to  the  difficulty  of  obtaining  and  preparing 
suitable  material,  and  it  is  proposed  to  show,  so  far  as  possible 
within  brief  limits,  that  the  collection  of  material  is  within  the 
reach  of  every  visitor  to  the  seaside,  and  that  the  subsequent 
preparation  presents  no  unusual  difficulty  to  the  microscopist. 

The  chief  sources  from  which  foraminifera  may  be  obtained 
are  : 

1.  Dredged  material,  including  anchor  muds  and  sands. 

2.  Shore  gatherings  made  between  tide  marks. 

3.  Sands,  clays,  and  limestones  of  various  geological  ages, 
especially  from  cretaceous  and  tertiary  deposits. 

As  probably  very  few  readers  will  have  the  opportunity  of 
dredging  for  material,  and  as  anchor  muds,  which  often  contain 
an  abundance  of  shallow-water  forms,  are  rarely  obtainable, 
owing  to  the  strange  reluctance  of  seamen  to  lend  themselves  to 
the  collection  of  scientific  material,  it  is  not  proposed  to  enter  at 
any  length  into  the  methods  of  collection  by  means  of  the  dredge. 
The  method  of  preparation  for  materials  of  this  class  is  essentially 
the  same  as  that  for  shore  gatherings. 

The  ordinary  naturalist's  dredge  can  be  successfully  used  for 

*  Reprinted  from  Knowledge  (with  additions). 

270 


COLLECTING  AND  PREPARING  FORAMINIFERA     271 

the  purpose  of  collecting  foraminiferous  material  from  the  sea- 
bottom,  but  if  the  dredge  is  of  the  usual  type,  with  the  bag  made 
of  rope-net,  it  will  be  necessary  to  insert  a  canvas  lining  to  the 
lower  end  of  the  bag,  in  order  to  insure  the  retention  of  some  of 
the  finer  sand  and  mud.  The  size  of  the  canvas  bag  must  be 
governed  by  the  strength  of  the  dredge  and  the  power  of  the  lift- 
ing gear.  A  large  deep-sea  dredge  such  as  is  used  on  scientific 
cruisers  will  have  a  mouth  4  or  5  feet  in  width  and  a  bag  6  feet 
long.  Such  a  dredge  will  on  a  soft  bottom  fill  up  in  a  quarter  of 
an  hour  or  less,  and  as  it  weighs  a  ton  or  more,  the  lifting  gear 
must  be  correspondingly  powerful.  When  the  dredge  is  operated 
and  lifted  entirely  by  hand,  quite  a  small  bag  in  the  end  of  the 
small  naturalist's  dredge  will  be  as  much  as  can  be  managed. 

After  the  dredge  has  been  emptied  on  board,  the  material  is 
usually  washed  in  a  large  tub  through  a  series  of  sieves  from 
jr  to  J  inch  mesh,  in  order  to  separate  the  mollusca  and  other  large 
organisms.  A  supply  of  the  mud  as  dredged  should  first  be  set 
aside  in  a  canvas  bag  and  labelled  '  Original  deposit.'  This 
would  be  required  for  any  quantitative  analysis  of  the  organisms 
contained  in  the  mud,  as  many  organisms  would  be  lost  during 
the  washing  process. 

After  selecting  the  softer  organisms  from  the  residue  left  on 
the  various  sieves  for  preservation  in  alcohol  or  formalin,  the 
residue  can  be  transferred  to  separate  canvas  bags  and  labelled 
with  particulars  of  the  sieve  from  which  it  was  obtained.  Large 
species  of  foraminifera,  and  especially  arenaceous  types,  will  be 
found  retained  on  the  ^-inch  sieve.  The  coarser  sieves  will  as  a 
rule  contain  only  stones  and  molluscan  or  echinoderm  fragments, 
but  these  will  often  be  found  to  be  covered  with  parasitic  fora- 
minifera. 

The  tub  in  which  the  material  has  been  washed  will  now  be 
found  to  contain  a  bottom  deposit  of  mud  and  sand  and  a  large 
quantity  of  muddy  water.  As  a  rule,  this  muddy  water  contains 
an  abundance  of  the  smaller  species  of  foraminifera  in  suspension, 
and  as  it  takes  a  long  time  for  them  to  settle  down  (in  rough 
weather  the  motion  of  the  ship  will  keep  them  constantly  in 
suspension),  the  muddy  water  should  be  baled  off  and  strained 
through  a  fine  silk  net,  such  as  a  tow-net.  By  this  means 
many  species  may  be  obtained  in  abundance  which  would  other- 


272  MODERN  MICROSCOPY 

wise  escape  observation.  After  the  water  has  drained  away 
through  the  net,  the  fine  mud  may  be  preserved  in  a  bag,  or, 
preferably,  in  alcohol.  Formalin,  having  an  acid  reaction, 
should  never  be  used  for  the  preservation  of  foraminifera. 

The  bottom  deposit  of  mud  from  the  tub  should  then  be  pre- 
served in  canvas  bags  for  preparation  ashore.  If  the  bags  are 
thoroughly,  but  slowly,  dried  over  the  engine-room,  and  stored  in 
a  dry  place,  the  material  can  be  preserved  for  many  years 
uncleaned  and  without  deterioration,  although  it  is  better  and 
easier  to  clean  it  as  soon  after  collection  as  possible. 

The  apparatus  required  by  the  shore-collector  is  of  the  simplest 
character,  and  consists  of  a  scraper  for  removing  the  surface  film 
of  sand,  which  alone  contains  foraminifera,  a  spoon  for  scraping 
material  from  ripple  marks  and  depressions,  and  a  metal  box, 
or  canvas  bag,  to  contain  the  gathering.  The  best  scraper  is  a 
thin  plate  of  celloidin  (about  the  thickness  of  a  visiting  card), 
such  as  a  '  photographic  '  film,  as  the  thinness  and  flexibility  of 
this  material  enables  the  collector  to  make  his  scraping  with  less 
admixture  of  sand  than  is  possible  with  the  glass  or  metal  slip 
usually  recommended  for  use. 

Thus  equipped,  the  collector  sallies  forth  between  the  tides. 
Probably  everyone  has  noticed  when  at  the  seaside  the  white 
lines  which  run  along  the  sands  parallel  with  the  retreating  tide. 
A  pocket  lens  shows  that  the  white  material  consists  largely  of 
the  minute  shells  of  foraminifera,  of  which  some  are  of  a  lustrous 
white  colour,  due  to  the  comparative  abundance  of  the  Miliolidae 
— a  family  of  common  occurrence  in  shore  gatherings,  char- 
acterized by  opaque  shells  of  a  milky  white  or  '  porcellanous  ' 
texture — while  others  are  more  or  less  glassy  and  transparent. 
These  '  hyaline '  forms  are  much  less  noticeable  to  the  naked 
eye.  They  are  mixed  in  varying  proportions  with  fragments  of 
shell  substance — ostracode  shells,  cinders,  and  the  lighter  debris 
of  the  shore — and  their  presence  in  these  lines  is  due  to  the 
separating  action  of  the  water,  which  on  a  smaller  scale  we  shall 
later  on  employ  in  the  cleaning  of  our  collected  material.  The 
rocking  action  of  the  wave  on  the  extreme  edge  of  the  ebbing 
tide  keeps  these  shells  and  fragments  of  light  specific  gravity  in 
suspension  until  after  the  heavier  sand-grains  have  subsided, 
and  so  they  are  left  behind  in  the  ripple  marks  and  depressions 


COLLECTING  AND  PEEPAEING  FOEAMINIFEEA     273 

of  the  sand.  Sometimes  a  local  eddy  of  the  tide,  produced  by 
the  neighbourhood  of  a  projecting  rock,  or  of  groins  and  piers, 
causes  the  material  to  be  gathered  together  in  large  quantities, 
which  show  as  extensive  white  patches  on  the  sand,  and  prove 
a  real  gold  mine  to  the  collector,  who  will  then  obtain  more 
material  in  half  an  hour  than  he  could  gather  in  several  days 
from  the  ripple  marks. 

The  collector  must  not  conclude  that  there  are  no  foraminifera 
present  because  there  are  no  white  patches  to  be  seen,  but, 
remembering  the  way  in  which  these  patches  are  formed  of  the 
lighter  debris  of  the  shore,  must  look  for  foraminifera  wherever 
he  observes  that  such  debris  has  been  deposited. 

On  every  coast,  at  intervals  of  varying  distance,  there  are 
spots  which  appear  to  be  the  foci  of  the  local  tides  and  currents, 
and  here  the  material  will  be  found  in  the  greatest  abundance. 
These  points  will  soon  be  discovered,  and  may  be  worked  at 
every  tide,  but  they  vary  continually  with  the  set  of  the  tide 
and  wind,  so  that  a  spot  which  has  proved  rich  may  be  quit 3 
bare  the  next  year.  Thus,  in  October,  1896,  Bognor — always  a 
rich  collecting-ground — had  its  richest  point  to  the  west  of  the 
pier ;  while  in  September,  1901,  there  was  very  little  material 
obtainable  except  at  Felpham,  two  miles  to  the  east,  where  the 
beach  was  thick  with  debris. 

Having  found  the  material,  the  collection  is  quite  an  easy 
matter.  With  the  celluloid  scraper  at  an  angle  of  60°,  the  thin 
surface  film  of  foraminifera  and  debris  is  easily  scraped  into  a 
heap,  and  transferred  to  the  box  or  bag.  Great  care  must  be 
exercised  not  to  dig  down  into  the  sand,  for  nothing  but  a  heavy 
bag  will  result  from  this,  the  foraminifera  being  confined  to  the 
surface  layer.  The  material  thus  collected  may  be  either  cleaned 
at  once,  or,  after  being  slowly  dried — avoiding  great  heat — may 
be  packed  away  in  bottles  for  a  more  convenient  period. 

The  apparatus  required  for  the  cleaning  and  preparation  of 
the  dried  material  is  simple  and  inexpensive,  and,  if  desired, 
much  of  it  may  be  easily  improvised.  The  most  necessary 
articles  are  a  photographic  developing-dish  of  china,  quarter- 
or  half-plate  size  according  to  fancy,  sieves  of  different  sizes 
and  materials  according  to  the  collector's  pocket,  a  cylindrical 
glass  jar  with  a  lip,  and  without  airy  neck  or  constriction  at  the 

18 


274  MODEEN  MICROSCOPY 

top,  or  a  large  plain  glass  jug,  and  a  retort  stand  or  tripod, 
made  of  an  iron  ring  riveted  on  three  legs. 

The  sieves  can  be  made  by  any  coppersmith,  and  it  is  very 
convenient  to  have  a  series  of  varying  degrees  of  coarseness  ; 
but  for  the  beginner,  two  sieves  of  40  and  120  meshes  to  the 
inch  respectively  will  be  sufficient.  The  writer's  sieves  are  of 
copper,  4  inches  high,  4  inches  diameter  at  top,  sloping  to 
3  inches  diameter  at  the  bottom.  A  smaller  size,  made  of 
telescope-tubing  1J  inches  in  diameter  and  1  inch  deep,  is  very 
useful  for  washing  small  gatherings.  The  writer  also  uses 
Mr.  Heron- Allen's  method  of  serial  sieves  fitted  one  within 
another.  But  great  care  is  required  in  the  use  of  this  method, 
as  the  lowest  sieves,  having  the  finest  mesh,  speedily  become 
choked  with  material,  and  then  overflow,  causing  the  loss  of 
specimens.  Zinc,  which  is  cheaper  than  copper,  can  be  used 
for  the  sieves. 

The  wire  gauze,  which  can  be  obtained  from  any  large  iron- 
monger, varies  in  price  according  to  the  number  of  meshes  to 
the  inch,  ranging  from  a  few  pence  per  square  foot  to  four 
shillings  for  the  finest  obtainable,  which  has  120  meshes  to  the 
inch,  the  diameter  of  each  aperture  being  about  aw  inch.     If  a 
finer  sieve  than  this  is  required,  as  it  sometimes  may  be,  the 
size  of  the  aperture  may  be  reduced  by  silver-plating  the  gauze, 
or,  preferably,  by  the  use  of  silk  bolting  cloth,  which  may  be 
obtained  up  to  200  meshes  to  the  inch.     The  wire  gauze  must 
be  strained  tightly  over  the  sieve  and  soldered  neatly  to  the 
edge,  so  that  there  is  no  ledge  of  solder  inside  to  retain  un- 
washed material.     If  silk  is  used,  a  sieve  must  be  made  without 
a  bottom,  and  having  a  turned-back  edge  at  the  lower  end,  so 
that  the  silk  may  be  strained  across  and  secured  with  string  or 
a  rubber  band.     Very  useful  and  effective  sieves  can  be  made 
out  of  brass  tube  3  inches  diameter,  having  a  detachable  clamp- 
ing collar  operated  by  a  screw.     The  silk  is  strained  over  the 
tube  and  held  in  position  by  the  collar.     They  are  much  easier 
to  clean  than  the  usual  sieve,  but  are  expensive  to  make.     The 
most  useful  sizes  for  a  series  of  sieves  are,  in  my  opinion,  12, 
20,  40,  80,  120,  and  150  (silk)  meshes  to  the  inch. 

Before  cleaning   the  shore  material,  it  must  be  slowly  and 
thoroughly  dried.     It  should  then  be  passed  through  the  twelve- 


COLLECTING  AND  PKEPAEING  FORAMINIFERA     275 

mesh  sieve  to  remove  all  the  coarse  debris,  stones,  shells,  cinders, 
etc.  Few,  if  any,  of  the  British  shore  species,  except  parasitic 
forms,  will  be  found  in  this  coarse  residuum,  but  it  should  be 
looked  over  with  a  pocket  lens  for  these,  or  for  abnormally  large 
specimens.  In  some  dredged  materials  and  in  tropical  gatherings, 
however,  this  coarse  residuum  will  be  found  to  be  full  of  fora- 
minifera. 

The  material  which  has  passed  through  the  twelve-mesh  sieve 
consists  of  foraminifera  mixed  with  other  light  debris  and  a 
considerable  quantity  of  sand,  and  the  collector  must  now  pro- 
ceed to  eliminate  the  whole,  or  nearly  the  whole,  of  the  sand, 
and  as  much  as  possible  of  the  other  debris,  by  means  of  two 
operations — '  floating  '  and  '  rocking.'  If  the  quantity  of  material 
to  be  operated  upon  is  small,  it  may  be  treated  off-hand,  but  if 
there  is  much,  it  is  well  to  sift  it  out  into  varying  degrees  of 
fineness  by  passing  it  through  a  series  of  sieves.  This  will 
simplify  the  floating  operations  by  insuring  that  the  particles 
are  approximately  of  similar  weight. 

The  floating  operations  must  be  performed  at  a  sink,  and,  if 
possible,  in  daylight,  the  process  being  more  uncertain  by  arti- 
ficial light.  The  finest  sieve  (120  wire  or  150  silk)  is  thoroughly 
wetted  and  rested  on  the  tripod.  The  glass  jar  is  then  filled  with 
water  nearly  to  the  brim,  and  a  few  spoonfuls  of  sand  slowly 
poured  into  it.  If  the  material  is  coarse  the  sand  sinks  instantly, 
and  in  the  course  of  a  few  seconds  most  of  the  foraminifera 
follow  suit.  By  holding  the  jar  to  the  light  the  course  of  the 
falling  particles  can  be  followed,  and  at  the  proper  moment  a 
sudden  tilt  empties  the  whole  of  the  water  and  most  of  the 
foraminifera  into  the  sieve,  the  sand  and  the  heavier  '  forams  ' 
being  left  in  the  jar.  The  purity  of  the  material  in  the 
sieve,  which  is  usually  called  '  floatings,'  will  depend  upon  the 
skill  and  judgment  of  the  operator,  and  is  largely  a  matter  of 
practice.  The  residuum  in  the  jar  must  be  washed  out  into  a 
basin  for  further  treatment,  and  the  operation  repeated  with 
more  sand  and  water  until  the  whole  of  the  gathering  has  been 
treated.  The  time  allowed  for  subsidence  will  vary  with  the 
fineness  of  the  sand,  so  that  in  the  case  of  the  finest  sittings 
nearly  a  minute  may  be  required.  The  actual  time  can  only  be 
determined  by  watching  the  falling  material  in  a  strong  light. 


276 


MODEEN  MICROSCOPY 


In  the  case  of  very  fine  sand,  the  tension  of  the  surface  film 
of  water  is  so  great  that  the  sand  grains  float  almost  as  readily 
as  the  foraminifera.  This  difficulty  may  be  overcome  by  shaking 
up  the  contents  of  the  jar,  covering  up  the  top  with  one's  hand 
while  so  doing. 

If  it  is  desired  to  obtain  a  very  pure  gathering  the  jar  should 
be  stirred  up  and  allowed  to  settle  for  a  few  minutes.  The 
lighter  species  of  foraminifera  will  be  found  floating  and  adhering 
to  the  sides  of  the  jar  at  the  surface  of  the  water.  They  can  be 
removed  with  a  cigarette-paper  or  feather,  and  washed  off  into  a 
separate  small  sieve.     I  call  these  '  double  floats.' 

The  residuum,  which  had  been  set  aside  in  a  jar,  may  now  be 
treated  by  the  '  rocking '  process  for  the  separation  of  the  remain- 
ing foraminifera.  Taking  the  photographic  developing  dish  (or 
a  tin  tray  may  be  used  as  a  substitute),  enough  of  the  residuum 
is  placed  in  it  to  cover  the  bottom  to  a  depth  of  about  J  inch, 
and  covered  with  about  f  inch  of  water.  If  the  dish  is  then 
rocked  with  a  combined  up  and  down  and  circular  motion,  the 
foraminifera  will  rise  in  suspension  in  the  water,  and  by  a  little 
careful  manipulation  may  be  gathered  in  one  corner  of  the  dish. 
A  sudden  tilt  will  then  empty  them  with  the  water  into  a  sieve. 
The  operation  should  be  repeated  with  two  or  three  lots  of  water, 
and  the  material  left  in  the  dish  will  then  be  found  to  consist 
almost  entirely  of  sand.  The  material  left  in  this  second  sieve, 
known  as  '  washings,'  is  not  so  pure  as  the  '  floatings,'  for  it 
contains  a  large  percentage  of  broken  forms  and  shell  fragments, 
coal-dust,  and  other  debris.  It  may  be  further  purified,  if  desired, 
by  being  dried  and  '  floated  '  once  or  twice  in  the  glass  jar. 

If  the  floatings  thus  obtained  contain  much  animal  or  vegetable 
matter,  as  is  sometimes  the  case,  it  is  advisable  to  boil  them  in 
a  solution  of  caustic  potash.  This  will  not  damage  the  forami- 
nifera so  long  as  the  boiling  is  not  carried  on  too  long,  and  it 
effectually  removes  the  animal  matter,  which  otherwise  would 
encourage  fungoid  growths.  This  process  must,  however,  be 
used  with  great  caution  if  the  gathering  contains  arenaceous 
foraminifera.  All  trace  of  the  caustic  potash  must  be  removed 
by  frequent  washing. 

The  processes  already  described  are  intended  for  recent  sandy 
gatherings.     When  the  material  is  in  the  form  of  dredged  mud, 


COLLECTING  AND  PREPARING  FORAMINIFERA     277 

it  is  first  necessary  to  get  rid  of  the  finest  particles  of  this  mud 
for  if  the  water  is  turbid  it  becomes  very  difficult  to  judge  the 
right  moment   for   separating  the   floating  forams.     The   mud 
should  be  broken  up  into  small  lumps,  about  an  inch  cube,  and 
slowly  but  thoroughly  dried.     It  is  then  placed  in  a  basin  and 
covered  with  water,  which  rapidly  breaks  it  up  into  a  fine  mud 
Boiling  water  acts  most  quickly,  and  if  the  mud  was  very  '  sticky  ' 
when  dredged  the  addition  of  washing  soda  will  facilitate  the  clean- 
ing process.     Great  care  must,  however,  be  taken  not  to  expose 
your  silk  sieves  to  the  soda  solution  for  more  than  a  few  seconds 
and  to  wash  them  thoroughly  afterwards.      Such  specimens  as 
may  be  observed  floating  on  the  surface  of  the  water  may  be 
easily  removed  by  means  of  cigarette-papers,  which  are  placed 
on  the  surface  of  the  water.     The  forams  adhere  to  the  papers 
which  are   then   carefully  lifted   off  and   dried,  the   specimens' 
being  afterwards  brushed  off  into  a  tube.     Many  delicate  forms, 
which   would   almost   certainly   be    broken   in   the    subsequent 
processes,  may  thus  be  obtained  in  a  perfect  state. 

The  mud  remaining  in  the  basin  is  then  washed,  a  spoonful 
at  a  time,  by  placing  it  in  a  sieve  of  fine  silk  gauze,  through 
which  a  gentle  stream  of  water  from  the  tap  (or  preferably 
from  a  '  rose '  attached  to  the  tap  by  a  short  length  of  tubing)  is 
kept  running  until  all  the  fine  particles  have  been  removed.  The 
muddy  water  should  be  allowed  to  settle  in  a  bath,  and  the  solid 
residuum  scraped  out  and  thrown  away.  The  sandy  residuum 
left  in  the  sieve  should  be  thoroughly  dried,  and  is  then  ready 
for  examination  under  the  microscope,  or,  if  desired,  it  may 
be  further  reduced  in  bulk  and  purified  by  the  floating  and 
rocking  processes  already  described. 

Foraminifera  occur  in  marine  fossil  deposits  of  all  geological 
ages,  from  the  Cambrian  to  post-tertiary,  but  they  are,  as  a  rule, 
of  sparing  occurrence  until  we  reach  the  cretaceous  period.  The 
harder  chalks  and  limestones  can  only  be  studied  by  means 
of  thin  sections,  but  the  softer  chalks,  shales,  and  clays  may  be 
broken  up  by  drying  the  material  in  small  pieces  and  washing 
it  over  a  fine  sieve  in  the  manner  just  described.  Floatings  are 
seldom  procurable  from  fossil  deposits,  owing  to  the  weight  of 
the  specimens,  which  are  generally  more  or  less  infiltrated  with 
pyrites  or  other  mineral  matter. 


278  MODERN  MICROSCOPY 

Some  chalks  and  shales  which  resist  the  disintegrating  action 
of  water  after  being  dried  may  be  broken  up  by  the  action  of  a 
crystallizing  salt,  which  has  been  absorbed  in  a  fluid  state. 
Acetate  of  soda  has  the  most  rapid  action,  but  very  fair  results 
may  be  obtained  with  common  washing  soda.  The  material, 
after  being  broken  up  into  small  pellets,  is  dropped  into  a 
boiling  saturated  solution  of  the  salt,  and  kept  at  this  tempera- 
ture for  a  short  time  to  allow  of  penetration.  The  salt  is  then 
allowed  to  cool,  and  in  cooling  crystallizes,  the  formation  of  the 
crystals  breaking  up  the  outer  layer  of  the  material.  On  being 
warmed,  the  soda  dissolves  again  in  its  own  water  of  crystalliza- 
tion, and  the  crystallization  is  repeated  over  and  over  again 
until  the  lumps  are  broken  up.  The  resulting  mud  is  then 
washed  in  the  ordinary  way. 

The  best  foraminifera  from  the  chalk  are  those  obtained  from 
the  interior  cavities  of  hollow  flints.  They  are  often  in  the  most 
perfect  state  of  preservation,  and  the  chalk  in  these  cavities  being 
of  a  powdery  nature,  they  are  very  easily  cleaned. 

The  cleaned  material  should  be  sifted  into  varying  degrees 
of  fineness,  and  each  grade  kept  separately  in  a  tightly  corked 
tube,  noted  with  locality,  date,  and  any  details  as  to  the  species 
contained  in  it,  which  may  be  likely  to  be  useful  for  future 
purposes  of  reference.  If  the  material  has  been  properly  cleaned 
and  dried  it  can  be  kept  unaltered  for  an  indefinite  period, 
bat  if  put  away  damp  fungoid  growth  will  quickly  set  in. 
This  can  be  destroyed  and  the  material  sterilized  by  a  pro- 
longed soaking  in  spirit,  the  material  being  afterwards  dried 
once  more. 

To  examine  the  material  under  the  microscope,  a  picking-out 
tray  will  be  necessary.  This  is  made  by  covering  a  slip  of  card 
with  coarse  black  ribbed -silk,  the  ribs  running  longitudinally 
along  the  slip.  A  thin  wooden  ledge  must  be  glued  round  three 
sides  of  the  slip  to  prevent  the  forams  rolling  off  when  the  stage 
of  the  microscope  is  inclined  at  an  angle.  The  '  Stephenson ' 
form  of  binocular  is,  of  course,  the  most  desirable  for  the  work 
of  selection,  as  the  stage  being  always  horizontal,  the  specimens 
cannot  roll  out  of  the  field.  The  material  is  sprinkled  over  the 
slip,  and  the  ridges  of  silk  keep  the  forams  from  rolling  about. 
The  specimens  required  can  then  be  easily  selected  by  means  of 


COLLECTING  AND  PEEPAEING  FOEAMINIFEEA     279 

a  fine  sable  brush,  moistened  by  drawing  it  between  the  lips, 
and  transferred  to  a  prepared  cell  or  slip. 

The  best  fixative  for  mounting  foraminifera  is  gum  tragacanth, 
which  is  almost  invisible  when  dry,  being  quite  devoid  of  the 
objectionable  glaze  which  characterizes  gum  arabic.  It  is  also 
much  less  subject  to  variations  of  moisture  than  gum  arabic, 
which  alternately  contracts  and  expands  with  changes  of  weather, 
and  often  fractures  delicate  forms.  Powdered  gum  tragacanth 
should  be  used  in  the  preparation  of  the  mucilage.  Put  a  small 
quantity  of  the  powdered  gum  in  a  bottle  with  sufficient  spirit  of 
wine  to  just  cover  it.  Add  a  small  crystal  of  thymol  or  a  few 
drops  of  clove  oil,  or  oil  of  cassia,  as  an  antiseptic,  then  fill  the 
bottle  with  distilled  water  and  set  it  by  for  some  hours.  The 
gum  will  form  a  thick  mucilage,  and  may  be  used  of  varying 
thicknesses  according  to  the  size  of  the  foraminifera.  For  most 
forms  it  should  be  of  about  the  consistency  of  cream,  and  it 
may  be  used  liberally  in  mounting,  as  it  shrinks  very  much  in 
drying. 

The  same  gum  diluted  to  a  watery  consistency  can  be  used  as 
a  fixative  for  foraminifera  mounted  in  balsam.  If  the  slide  is 
thoroughly  dried  before  the  balsam  is  added  the  gum  becomes 
quite  invisible. 

For  very  large  and  heavy  foraminifera,  seccotine  or  some  other 
liquid  glue  may  be  used  with  advantage,  gum  not  being  of 
sufficient  strength  to  hold  them  safely. 

Many  fossil  foraminifera  and  recent  forms  from  some  localities 
have  the  internal  chambers  filled  with  mineral  infiltrations, 
either  glauconite  or  pyrites.  These  internal  casts  reproduce 
more  or  less  perfectly  the  shape  of  the  sarcode  body  of  the 
animal.  They  may  be  obtained  by  decalcifying  the  specimens 
with  very  dilute  nitric  acid,  just  faintly  acid  to  the  taste.  To 
obtain  perfect  casts  the  process  must  be  carried  out  very  slowly, 
adding  drop  by  drop  to  the  watch-glass  containing  the  specimen. 
When  decalcification  is  complete  the  resulting  cast  should  be 
carefully  removed  with  a  pipette,  and  deposited  in  a  spot  of  gum 
on  a  slip.  They  will  not  stand  transference  with  a  brush  without 
damage. 

Artificial  casts  of  the  animal  body  may  be  taken  in  paraffin 
wax,  by  a  method  suggested  by  the  writer  to  Mr.  H.  J.  Quilter, 


280  MODERN  MICROSCOPY 

a  nd  described  by  him  in  the  Journal  of  the  Quekett  Microscopical 
Club  for  1903. 

Briefly,  the  method  as  improved  by  subsequent  experience  is  as 
follows :  (1)  Boil  the  foraminifera  in  weak  caustic  potash  to  remove 
animal  matter,  wash  thoroughly  and  dry.  (2)  Soak  in  chloroform 
for  a  few  days,  until  all  the  air  is  expelled  from  chambers. 
(3)  Drop  straight  from  chloroform  into  melted  paraffin  wax  of  high 
melting-point,  and  keep  the  wax  hot  over  a  spirit  lamp  until  all 
the  chloroform  has  been  expelled  as  bubbles.  (4)  Then  remove 
the  foraminifera  with  a  forceps  or  brush  to  cover-glasses,  and 
remove  any  excess  of  paraffin  wax  with  blotting-paper.  (5)  After 
the  wax  cools,  the  foram  is  attached  to  the  cover  by  a  film  of 
wax.  Clean  the  top  surface  of  specimen  by  rubbing  it  with 
a  fragment  of  blotting-paper  dipped  in  xylol.  This  operation 
should  be  done  under  the  microscope,  as  it  is  only  desired 
to  remove  the  surface  film  of  wax.  (6)  Drop  the  cover-glass 
into  a  beaker  of  strong  nitric  acid.  The  rapid  effervescence 
tears  the  outer  film  of  wax  apart,  and  the  internal  cast  floats 
free.  It  may  then  be  taken  up  carefully  on  a  brush  and 
mounted  dry. 

The  process  is  a  most  interesting  one,  and  the  results  with  the 
larger  species  are  surprisingly  beautiful.  The  smaller  species 
are,  however,  very  difficult  to  manage  satisfactorily. 

Sections  of  foraminifera  may  be  made,  and  will  be  found  very 
useful,  and  sometimes  indispensable,  for  the  study  of  their 
internal  structure.  If  it  is  merely  desired  to  lay  the  shells  open 
in  order  to  show  the  interior  chambers  as  an  opaque  object,  this 
may  be  done  by  fixing  them  on  a  slip  or  cover-glass  and  rubbing 
lightly  on  a  fine  hone  or  on  a  sheet  of  fine  ground  glass.  The 
risk  of  fracture  is  greatly  lessened  if  the  chambers  are  first 
infiltrated  with  wax  or  with  Canada  balsam  in  the  same  manner 
as  in  the  preparation  of  casts,  the  matrix  being  removed  with 
xylol  after  the  section  has  been  made.  If,  however,  it  is  desired 
to  cut  a  transparent  section,  the  chambers  must  be  infiltrated 
with  toughened  balsam.  After  soaking  in  chloroform  until  all 
air  is  got  rid  of,  the  specimens  should  be  placed  in  a  watch- 
glass  and  covered  with  balsam  in  chloroform.  This  must  be  left 
for  a  day  or  two,  then  toughened  in  a  cool  oven.  If  the  balsam 
is  then  melted  the  specimens  can  be  removed  to  slips,  where 


COLLECTING  AND  PREPARING  FORAMINIFERA     281 

they  will  be  fixed  on  cooling,  and  are  then  ready  to  be  ground 
down.  When  ground  to  the  required  depth  the  slide  is  again 
warmed,  the  specimen  turned  over,  and  the  grinding  repeated 
with  extreme  care  until  the  section  is  reduced  to  the  required 
thickness.  The  operation  is  one  of  the  most  extreme  delicacy, 
and  no  microscopist  need  expect  to  succeed  without  preliminary 
failures  innumerable.  A  single  movement  on  the  hone  beyond 
the  requisite  point,  generally  suffices  to  wipe  the  specimen  out  of 
existence. 


CHAPTEE  XXV 

NOTES  ON  THE  COLLECTION,  EXAMINATION,  AND 
MOUNTING  OF  MOSSES  AND  LIVERWOETS 

By  T.  H.  EUSSELL,  F.L.S., 
Author  of  'Mosses  and  Livenvorts.' 

Collection  of  Specimens. — The  appliances  for  the  collection  of 
specimens  are  simple  in  the  extreme.  For  many  years  I  have 
been  in  the  habit  of  putting  the  material  gathered  in  the  field 
into  old  envelopes  that  have  been  cut  open  at  the  narrow  end 
instead  of  at  the  side.  Not  only  do  these  form  most  convenient 
pockets  for  the  purpose,  but  notes  can  be  made  on  them  at 
the  time,  of  the  date  and  locality  when  and  where  the  plants 
were  found,  and  of  the  names  that  suggest  themselves  on  a  first 
inspection  (a  useful  aid  to  the  cultivation  of  the  power  to  recog- 
nize plants  in  the  field).  Rough  memoranda,  too,  may  be  added 
afterwards  of  any  special  features  of  interest  that  present  them- 
selves on  a  closer  examination  of  the  contents,  and  that  need 
elucidation.  Squares  of  stout  paper  will  be  equally  serviceable, 
and,  indeed,  some  of  these  should  always  be  included  for  the 
putting  up  of  larger  plants.  They  will  also  prove  more  con- 
venient when  specially  wet  material  has  to  be  stored,  as  the  only 
objection  to  the  use  of  the  envelopes  is  that  moisture  is  apt  to 
loosen  the  gum  which  fastens  the  several  portions  together. 

A  magnifying-glass  with  a  fairly  large  field,  for  making  a 
preliminary  acquaintance  with  the  gatherings,  and  for  exploring 
purposes,  will  also  be  required.  I  always  carry  a  second  glass 
of  a  higher  magnifying  power — e.g.,  Browning's  platyscopic  lens 
with  a  magnification  of  ten  diameters — which  is  often  most 
helpful  when  minute  details  have  to  be  ascertained.     An  old 

282 


MOSSES  AND  LIVERWORTS  288 

knife,  with  which  to  dislodge  the  plants  and  to  free  them  from 
soil  and  grit ;  a  lead-pencil  for  making  notes  of  habitats,  etc. ; 
and  a  satchel,  to  serve  as  a  receptacle  for  the  envelopes  and 
papers,  both  when  empty  and  filled,  complete  the  only  real 
requirements  of  the  moss-hunter. 

On  reaching  home  the  envelopes  that  have  been  used  should 
be  placed  in  a  warm  room,  in  an  upright  position,  and  with  the 
ends  open  as  widely  as  possible  so  as  to  admit  air,  and  the 
packets  unfolded  ;  this  will  allow  the  specimens  inside  to  dry, 
a  matter  of  no  small  moment  if  the  risk  of  mildew  is  to  be 
avoided.  After  standing  thus  for  a  few  da}-s  they  can  be  safely 
put  away  until  a  convenient  opportunity  occurs  for  examining 
the  contents,  when  soaking  in  hot  water  will  speedily  restore  the 
plants  to  their  original  freshness,  though  not,  of  course,  to  life. 
The  small  china  saucers,  made  in  different  sizes,  to  be  pro- 
cured from  any  artists'  colourman,  are  very  convenient  for  this 
purpose.  The  ease  with  which  mosses  can  in  this  way  be  revived 
for  examination  constitutes,  to  my  mind,  one  of  the  chief  attrac- 
tions which  this  branch  of  botanical  research  offers  to  anyone  in 
search  of  a  hobby,  for  while  the  gathering  of  specimens  forms  a 
healthy  outdoor  occupation  for  all  seasons  of  the  year,  and  adds 
immensely  to  the  pleasure  and  interest  of  a  ramble  in  the  country, 
their  examination  and  mounting  may  be  deferred  for  any  length 
of  time,  and  will  provide  the  most  pleasurable  recreation  by  the 
fireside  in  the  long  winter  evenings. 

Preparation  of  Specimens. — To  prepare  specimens  for  ex- 
amination or  for  mounting,  some  form  of  dissecting  microscope 
is  practically  a  necessity.  For  many  years  I  used  an  ordinary 
magnifying-glass  of  low  power,  mounted  in  a  light  metal  frame, 
at  one  end  of  which  is  a  small  collar,  which  slips  over  a  screw 
fixed  in  an  upright  position  in  a  small  metal  stand,  and  provided 
with  a  nut  by  means  of  which  the  lens  can  be  adjusted  to  any 
required  height,  and  this  simple  expedient  is  still  often  very 
serviceable. 

Another  inexpensive  and  useful  instrument  for  the  purpose 
is  the  ordinary  watchmaker's  glass,  consisting  of  a  lens  set 
in  a  deep  horn  mount,  by  the  help  of  which  it  can  be  retained 
in  position  in  front  of  the  eye ;  but  care  must  be  taken, 
especially  when  this  is  used  in  mounting,  that  the  muscles  are 


284  MODERN  MICROSCOPY 

not  unconsciously  relaxed  in  the  interest  of  the  work,  and  the 
glass  allowed  to  fall.  As  a  rule,  however,  I  now  employ  the 
more  modern  binocular  dissecting  microscope,  which  is  also  of 
the  greatest  assistance  in  mounting. 

For  dissecting  purposes  ordinary  sewing  needles  set  in  cedar 
penholders  are  frequently  very  handy,  and  it  is  well  to  have 
one  or  two  bent  at  an  angle  to  the  holder,  as  these  will  be 
invaluable  in  mounting,  both  for  the  purpose  of  altering  the 
position  of  objects  after  the  cover-glass  has  been  put  on  and 
for  removing  stray  bubbles  of  air.  In  order  to  bend  a  needle 
into  almost  any  form,  it  is  only  necessary  to  heat  it  in  a  spirit- 
lamp  to  a  red  heat,  and  then  allow  it  to  cool ;  this  will  render 
it  soft  and  pliable.  After  being  bent  to  the  required  form,  it 
can  be  rehardened  by  plunging  it  when  red-hot  into  water. 

While  dissecting  instruments  of  various  kinds  can  be  purchased, 
I  know  of  none  that  better  serve  the  purpose  than  such  as 
may  easily  be  constructed  by  the  use  of  sail  needles  and  glovers' 
needles  (No.  4)  ;  the  former  for  ordinary  work  and  the  latter  for 
the  more  delicate  operations.  Sail  needles  can  be  obtained  at  an 
ironmonger's  and  glovers'  needles  from  a  draper.  The  needle, 
in  addition  to  having  a  fine  point,  is  ground  with  three  flat  faces, 
which  give  as  many  cutting  edges.  In  the  case  of  the  sail  needles 
these  edges  are  somewhat  blunt,  and  the  needle  should  therefore 
be  sharpened  on  a  hone  before  being  used.  Cheap  black-lead 
pencils  make  very  good  handles  in  which  to  mount  the  needles. 
After  being  cut  into  suitable  lengths,  they  should  be  well  soaked 
in  hot  water ;  this  melts  the  glue  that  fastens  the  two  portions 
of  the  wood  together,  and  the  lead  can  then  be  easily  removed. 
The  needle  should  be  placed  in  the  groove  thus  provided  for  it, 
and  should  be  allowed  to  project  some  little  way  beyond  the  end  of 
the  wood.  A  fine  pin  or  needle  is  then  run  through  the  eye,  and 
is  cut  off  with  a  pair  of  wire-nippers,  leaving  just  enough  to 
press  into  the  other  half  of  the  holder,  when  put  into  place. 
Some  amount  of  packing  may  be  necessary,  especially  with  the 
glovers'  needles,  to  keep  the  needle  rigid.  The  two  parts  of  the 
holder  are  then  fastened  together  with  liquid  glue  or  seccotine, 
each  end  being  afterwards  tied  round  with  fine  twine,  as  a  still 
further  precaution.  It  is  well  to  scrape  off  all  traces  of  the  glue 
from  the  outside  of  the  holder,  as  otherwise  it  is  apt  to  stick 


MOSSES  AND  LIVEEWOETS  285 

unpleasantly  to  the  lips  if  held  there  for  a  minute  in  the  hurry  of 
mounting.  Two  pairs  of  forceps  (one  with  curved  ends),  a  pair 
of  small  scissors,  a  small  camel's-hair  brush,  and  one  or  two  small 
lancets,  will  practically  complete  the  implements  required  for  all 
ordinary  dissecting,  to  which  must  be  added,  for  the  purpose  of 
microscopical  examination  and  subsequent  mounting,  a  stock  of 
the  usual  glass  slips  (3  inches  by  1  inch),  and  cover-glasses  of  two 
or  three  different  sizes.  Though  the  round  cover-glass  is  generally 
to  be  recommended,  both  on  the  ground  of  appearance  and  of 
ease  in  sealing,  yet  with  many  of  the  larger  mosses  the  square 
form  will  prove  more  serviceable,  as  it  gives  more  mounting 
surface,  while  for  dealing  with  plants  of  any  considerable  size 
specially  large  pieces  of  cover-glass  may  have  to  be  procured. 

Mounting. — Owing  to  their  small  size  and  the  facility  with 
which  their  original  freshness  can  be  revived,  as  already  noticed, 
mosses  can  be  far  more  satisfactorily  preserved  than  is  possible 
with  ordinary  flowering  plants.  The  greater  number  may  be 
readily  mounted  on  the  ordinary  glass  slips,  and  in  this  form 
they  not  only  occupy  a  comparatively  small  storage  space,  but 
remain,  for  all  practical  purposes,  as  fresh  as  when  they  were 
gathered.  I  have  specimens  in  my  collection  now  that  were  put 
up  twenty  years  and  more  ago,  and  which  have  altered  little  in 
appearance  in  the  meantime. 

I  have  tried  several  materials  and  compounds  for  mounting 
purposes,  but  unhesitatingly  give  the  palm  to  glycerine  jelly, 
both  on  account  of  the  ease  with  which  it  may  be  manipulated 
and  by  reason  of  its  admirable  preservative  powers.  I  shall, 
therefore,  mainly  confine  myself  to  a  description  of  the  method 
of  procedure  when  this  medium  is  used.  I  have  for  many  years 
made  my  own  jelly  according  to  the  following  recipe,  which  is 
a  slight  variation  on  that  given  in  Carpenter's  work  on  "  The 
Microscope."  Take  2  ounces  (by  weight)  of  the  best  gelatine, 
6  ounces  of  water  (also  by  weight),  and  6  of  glycerine.  Soak  the 
gelatine  in  the  water  until  it  swells  (this  takes  about  forty 
minutes) ;  then  place  the  vessel  containing  the  gelatine  and  water 
(a  jam-pot  is  very  serviceable ;  it  should  be  provided  with  a  cove.* 
of  some  kind)  in  a  saucepan  of  water,  and  boil  over  a  slow  tire 
until  the  gelatine  melts.  When  the  gelatine  is  cool,  but  still 
liquid,  add  the  white  of  one  egg,  and  mix  well.     Boil  the  gelatine 


286  MODERN  MICROSCOPY 

as  before,  until  it  becomes  thick  with  the  coagulated  albumin — 
this  takes  about  twenty  minutes ;  add  the  glycerine  and  25  or  30 
drops  of  carbolic  acid  and  mix  well ;  strain  through  filter-paper 
before  the  fire,  and  a  clear  pale  yellow  jelly  should  be  the  result. 

The  specimen  to  be  mounted  must  first  be  cleansed  from  all 
earth  and  grit  in  water,  the  spores  gently  expelled  from  any 
capsules,  and  all  air-bubbles  removed  by  means  of  the  dissecting 
needles,  and  too  much  care  cannot  be  given  to  this  somewhat 
tiresome  process,  as  on  its  due  performance  the  success  of  the 
mount,  to  a  very  large  extent,  depends.  But  our  moss  is  not  yet 
ready  to  be  put  into  the  jelly,  for  if  this  were  straightway  to  be 
done,  the  effect  would  inevitably  be  that  the  leaves  would  curl 
up  beyond  recognition.  A  simple  plan  for  guarding  against  this 
is  to  boil  the  plant  for  a  few  seconds  over  a  spirit-lamp,  in  a 
teaspoonful  of  water,  in  which  three  or  four  drops  of  glycerine 
have  been  mixed ;  but  unless  time  is  of  moment,  it  is  better  to 
leave  it  to  soak  for  twenty-four  hours  in  a  mixture  composed  of 
water,  1  \  ounces  (fluid) ;  rectified  spirit,  \\  ounces;  and  glycerine, 
5  drachms.  The  small  china  pans  in  which  moist  water  colours  are 
sold  are  very  useful  for  this  purpose.  Not  unfrequently  I  subject 
it  to  both  treatments,  as  this  tends  more  thoroughly  to  remove 
all  air.  When  taken  from  this  preparatory  bath  care  must  be 
taken  to  remove  the  fluid  adhering  to  the  plant  as  far  as  possible  ; 
this  may  be  done  by  placing  it  on  a  glass  slip,  and  tilting  this  so 
as  to  allow  the  superfluous  liquid  to  drain  off,  with  possibly  a 
judicious  application  of  blotting-paper,  though  this  must  be  used 
with  caution,  as  otherwise  fresh  air  is  apt  to  be  admitted. 

A  hot-water  bath  is  essential  for  mounting  with  glycerine  jelly. 
A  simple  and  inexpensive  one  can  be  made  by  means  of  a  small 
glass  tumbler,  provided  with  a  closely  fitting  tin  cap  or  lid,  having 
a  piece  cut  out  of  the  margin,  leaving  just  room  enough  to  admit 
the  neck  of  a  small  glass  bottle  containing  the  jelly.  The  bottle 
can  thus  hang  in  the  hot  water  in  the  tumbler,  when  the  lid  is  in 
place,  by  means  of  its  projecting  lip,  which  rests  on  the  top  of  the 
tin  cover,  and  in  this  way  the  jelly  is  kept  melted,  and  is,  more- 
over, close  at  hand  for  use.  The  advantage  of  having  a  small 
bottle  for  this  purpose  (which  is,  of  course,  replenished  from 
time  to  time  from  the  larger  stock  bottle)  is  that  the  necessity  is 
obviated  of  continually  remelting  the  same  jelly,  which  would 


MOSSES  AND  LIVERWORTS  287 

cause  undue  evaporation  of  some  of  its  component  parts.  When 
my  mounting  is  likely  to  take  long  I  wrap  a  piece  of  flannel 
round  the  tumbler,  in  order  to  retain  the  heat  in  the  water  as 
long  as  possible.  The  glass  slip  on  which  the  mount  is  to  be 
made,  as  also  the  cover-glass,  must  be  first  carefully  cleaned.  A 
good  plan  is  to  rub  them  over,  between  the  finger  and  thumb, 
with  acetic  acid,  in  order  to  remove  all  traces  of  grease,  and 
then  to  wash  them  in  warm  water,  afterwards  drying  with  a  soft 
cambric  handkerchief,  a  final  polish  being  given  with  a  wash- 
leather.  The  glass  slip  is  now  placed  upon  the  flat  tin  cover  of 
the  hot-water  bath,  a  small  pool  of  the  liquid  jelly  is  put  upon  it 
by  means  of  a  pipette,  and  the  specimen,  after  being  freed  from 
the  preparatory  fluid,  is  gently  lowered  into  the  jelly.  WThile  the 
latter  is  kept  liquid  by  the  heat  of  the  water-bath,  all  air-bubbles 
must  be  carefully  removed  with  the  dissecting  needle,  and  here  the 
binocular  dissecting  microscope  will  be  found  most  helpful. 

It  is  impossible  to  exaggerate  the  importance  of  this  extraction 
of  air,  for  nothing  detracts  more  from  the  appearance  of  a  mount 
when  viewed  under  the  microscope  than  the  presence  of  these 
disfiguring  silvery  globes,  lurking  among  the  delicate  leaves,  or 
perhaps  in  the  teeth  of  the  peristome  ;  and  my  own  rule  always 
is  that,  rather  than  allow  a  serious  blemish  of  this  kind,  the  slide 
must  be  sacrificed  or  the  mount  be  recommenced.     I  have  found 
it  a  great  help  in  many  cases,  especially  when  an  object  likely 
to  retain  air  or  an  undue  amount  of  the  preparatory  fluid  is  in 
hand — as,  for  instance,  a  large  empty  capsule  or  a  plant  with 
the  leaves  closely  covering  the  stem — to  put  it  into  a  little  jelly 
on  a  spare  glass  slip,  and  then  to  extract  the  air  as  far  as  possible 
before  transferring  it  to  the  slip  on  which  it  is  to  be  mounted. 
The  whole  plant  thus  becomes  more  or  less  saturated  with  the 
melted  jelly,  and  the  air-bubbles  cannot  find  their  way  back  to  the 
mount,  as  they  are  apt  to  do  if  the  whole  process  is  carried  out 
on  the  one  slide.     A  second  hot-water  bath  is  then  a  practical 
necessity,  in  order  that  the  jelly  thus  left  in  contact  with  the 
specimen  may  not  solidify  before  the  latter  is  transferred  to  the 
glass  slip.    When  everything  has  been  prepared,  and  the  specimen 
is  in  place,  immersed  in  plenty  of  the  liquid  jelly,  the  cover-glass 
is  taken  up  with  the  forceps,  and  gently  lowered  on  the  jelly, 
beginning  from  the  left-hand  side,  driving  the  jelly  (and  too  often, 


288  MODERN  MICROSCOPY 

alas  !  the  specimen  also)  before  it,  as  it  is  allowed  gradually  to 
fall  into  place.  This  is  an  operation  of  no  little  delicacy,  as  if 
great  care  is  not  taken  a  large  bubble  of  air  will  make  its  way  in 
at  the  last  moment.  If,  as  frequently  happens,  the  putting  on 
of  the  cover-glass  has  caused  a  displacement  of  the  object,  this 
must  be  rectified  before  the  jelly  is  allowed  to  set,  and  here  the 
bent  dissecting  needles  will  be  of  great  service,  as  a  considerable 
amount  of  rearrangement  can  be  effected  with  one  of  them,  and 
stray  air-bubbles  may  also  be  removed  without  disturbing  the 
cover-glass. 

Should  it  happen  that  not  quite  enough  jelly  has  been  used 
to  fill  the  whole  of  the  space  under  the  cover-glass,  a  small 
additional  quantity  must  be  introduced  by  means  of  the  pipette, 
care  being  taken  to  avoid  the  formation  of  air-bubbles.  The  slide 
is  now  taken  from  the  hot- water  bath  and  is  allowed  to  cool,  and 
in  a  few  minutes  the  jelly  will  have  so  far  solidified  that  it  can 
be  examined  under  the  microscope,  when,  should  any  serious 
defect  be  disclosed,  the  jelly  must  be  remelted,  and  the  short- 
coming be  rectified.  The  final  process  consists  in  removing  the 
superfluous  jelly  from  around  the  cover-glass  with  a  knife, 
cleaning  the  slide  from  all  trace  of  the  jelly  (a  handkerchief 
moistened  at  the  lips  is  the  most  efficient  method),  and  sealing 
the  cover-glass  round  the  margin  with  some  kind  of  varnish. 

I  may  add  that  I  usually  mount  two  cells  on  each  slide ;  in 
the  larger  of  them  I  place  a  small  portion  of  the  moss,  together 
with  a  few  capsules,  if  possible  in  various  stages  of  growth,  and 
two  or  three  perichaatial  leaves,  while  the  other  cell  contains 
some  leaves  dissected  from  the  plant  (wThere  of  importance  from 
both  stem  and  branch),  and  a  few  pieces  cut  from  the  mouth  of 
the  capsule,  to  show  the  peristome. 

It  will  be  very  evident  that  the  subject  of  the  removal  of  air 
has  frequently  cropped  up  in  what  I  have  written ;  this  is 
accounted  for  by  the  fact  that  it  betokens  the  chief  difficulty  to 
be  encountered  in  mounting  w7hen  glycerine  jelly  is  the  selected 
medium.  There  is,  unfortunately,  no  royal  road  to  success  in 
this  particular,  and  the  main  thing  to  rely  on  is  a  patient  use  of 
the  dissecting  needles.  One  or  two  hints  as  to  matters  of  detail 
may,  however,  be  given.  Boiling  the  specimen,  more  especially 
if  it  be  old,  is  often  helpful ;  and  soaking  it  in  water  that  has 


MOSSES  AND  LIVERWOETS  289 

been  allowed  to  boil  for  ten  minutes  is  sometimes  recommended. 
In  this  connection,  too,  I  may  mention  that  it  is  important  to 
keep  the  pipette,  used  for  abstracting  the  melted  jelly  from  the 
bottle,  quite  clean,  as,  if  earthy  matter  is  allowed  to  get  encrusted 
on  it,  small  bubbles  of  gas  are  apt  to  be  formed,  and  these  easily 
get  transferred  to  the  jelly  itself.  I  have  also  found  that  atten- 
tion to  another  seemingly  trivial  point  is  of  no  little  moment, 
and  that  is,  when  dipping  the  pipette  into  the  melted  jelly  always 
keep  a  finger  on  the  open  end  until  the  point  has  reached  the 
bottom  of  the  bottle.  As  heat  naturally  causes  any  bubbles  in 
the  jelly  to  rise  to  the  surface,  this  minimizes  the  risk  of  their 
entering  the  pipette,  for  the  lower  strata  of  jelly  are,  so  to  speak, 
tapped. 

Glycerine  Jelly. — As  far  as  my  acquaintance  with  glycerine 
jelly  goes — and  it  is  one  of  a  good  many  years'  standing — the 
great  objection  to  its  use  arises  from  the  fact  that  after  a  speci- 
men has  been  mounted  in  it,  and  has  stood  possibly  for  years, 
the  jelly  may  develop  an  unpleasant  tendency  to  liquefy,  as  will 
be  evidenced  by  the  presence  of  small  beads  of  glycerine  round 
the  edge  of  the  cover-glass. 

While  not  being  able  to  suggest  any  unfailing  remedy  for  evils 
such  as  this,  I  have  noticed  that  the  observance  of  a  few  simple 
rules  will  considerably  minimize  the  risk.     These  are — 

1.  Care  should  be  taken  to  remove  the  preparatory  fluid  from 
the  surface  of  the  object  as  far  as  possible  before  mounting, 
without,  of  course,  running  too  much  risk  of  admitting  air. 

2.  No  pressure  should  be  applied  to  the  cover-glass  in  order 
to  keep  it  in  position,  its  own  weight  being  alone  sufficient  for 
the  purpose. 

3.  A  sufficient  quantity  of  the  jelly  should  be  used  to  allow  of 
a  small  portion  extending  on  every  side  beyond  the  edge  of  the 
cover-glass,  in  order  to  provide  against  subsequent  shrinkage  in 
the  gelatine. 

4.  The  slide  should  stand  for  at  least  two  or  three  months 
before  the  additional  jelly  is  removed  and  the  cell  sealed. 

5.  The  slides  should  be  kept  in  a  fairly  equable  temperature, 
and  should  not  be  exposed  to  draught. 

I  generally  add  a  very  small  amount  either  of  carbolic  acid, 

or  of  a  5  per  cent,  solution  of  bichloride  of  mercury  (corrosive 

19 


290  MODERN  MICROSCOPY 

sublimate)  to  the  liquid  jelly  on  the  glass  slip,  before  the  speci- 
men is  put  into  it,  as  this  lessens  the  chance  of  any  fungoid 
growth  subsequently  developing  in  the  cell.  It  will  suffice  if  the 
tip  of  a  thin  glass  rod  that  has  been  drawn  to  a  point  is  just 
dipped  into  the  liquid,  the  small  amount  clinging  to  it  being 
transferred  to  the  jelly,  and  quickly  mixed  with  it.  In  the  case 
of  the  bichloride  of  mercury  it  is  especially  important  not  to 
introduce  too  much,  or  a  slight  precipitate  of  calomel  will  result, 
giving  a  cloudy  appearance  to  the  jelly. 

Formalin. — A  3  per  cent,  solution  of  formalin  is  also  a  most 
serviceable  mounting  medium,  more  particularly  where  some  of 
the  more  delicate  plants  are  under  treatment,  though  it  is,  of 
course,  open  to  the  objections  that  attach  to  the  use  of  all  liquid 
media.  If  only  leaves  are  being  mounted — and  it  is  for  such  a 
purpose  that  the  solution  is  most  suitable — a  cell  of  some  spirit 
varnish  must  first  be  made  by  means  of  a  turn-table.  When  this 
is  dry  the  top  of  the  cell  is  ringed  round  with  marine  glue  dis- 
solved in  benzol  (as  to  which,  more  hereafter),  and  sufficient  of 
the  solution  is  introduced  into  the  cell  from  a  pipette  to  allow  of 
its  standing  well  above  the  walls  of  the  cell.  The  object  is  now 
carefully  introduced,  the  cover-glass  is  taken  between  the  finger 
and  thumb,  and,  after  being  brought  as  near  to  the  solution  as 
possible,  is  allowed  to  fall  gently  into  place.  Should  air  have 
unluckily  made  its  way  in,  the  cover-glass  must  be  quickly  raised, 
and  a  little  more  of  the  solution  be  introduced.  The  cover-glass 
is  now7  gently  pressed  down  on  to  the  top  of  the  cell,  and,  after 
all  superfluous  moisture  has  been  removed  with  a  handkerchief, 
is  ringed  round  with  the  marine  glue  solution,  and  afterwards 
with  spirit  varnish  ;  being  finished  off,  if  thought  desirable,  with 
a  coat  of  asphalt  varnish.  If  a  larger  object  is  to  be  mounted  a 
deeper  cell  must,  of  course,  be  used. 

Varnishes. — I  have  tried  a  good  many  sealing  materials,  and 
on  the  whole  much  prefer  picture  copal  varnish  (to  be  obtained 
from  any  artists'  colourman)  thinned  with  benzol.  It  does  not 
dry  too  hard,  and  in  consequence  is  not  liable  to  crack ;  while, 
should  any  portion  of  the  object  happen  to  be  located  near  to 
the  edge  of  the  cover-glass,  it  can  nevertheless  be  seen  through 
the  practically  colourless  varnish.  Where  the  object  is  of  any 
size,  such  as  a  piece  of  one  of  the  larger  plants,  it  becomes 


MOSSES  AND  LIVERWORTS  291 

advisable  to  use  a  sealing  medium  that  will  adhere  more 
tenaciously  to  the  sides  of  the  cell,  and  here  nothing  serves 
the  purpose  better  than  marine  glue  dissolved  in  benzol.  I 
should  advise  anyone  who  intends  to  employ  it  to  get  the 
preparation  ready-made  rather  than  attempt  its  manufacture, 
for  if  there  is  one  medium  that  is  more  exasperatingly  adhesive 
and  sticky  than  another  it  is  marine  glue.  It  is,  however,  a 
very  safe  material  to  use,  and  is,  as  far  as  my  experience  goes, 
quite  free  from  any  tendency  to  '  run  in,'  which  I  have  always 
found  to  be  the  shortcoming  of  gold  size.  The  cell  may  be 
finished  later  with  asphalt  varnish. 

Although  I  have  throughout  referred  to  mosses  alone,  yet  the 
methods  of  which  I  have  spoken  are  equally  applicable  to  the 
mounting  of  liverworts.  The  only  additional  point  to  be  men- 
tioned with  regard  to  the  latter  is  this  :  Owing  to  the  fact  that 
the  contents  of  the  leaf-cells  are  specially  dense,  it  is  often 
advisable,  in  order  to  render  the  cell-structure  more  distinct, 
to  treat  some  leaves  with  a  strong  alkali.  The  best  way  is  to 
place  a  piece  of  the  plant  in  a  few  drops  of  a  7  per  cent,  solution 
of  liquor  potassae  on  a  glass  slip,  to  cover  this  with  a  cover-glass, 
and  then  to  boil  over  a  spirit-lamp.  The  plant  will  need  to  be 
well  cleansed  by  boiling  in  water  before  the  leaves  are  mounted 
and  a  few  of  them  may  then  be  conveniently  included  in  one  of 
the  cells. 


CHAPTEK  XXVI 
THE  MICROSCOPE  AND  NATURE  STUDY 

By  WILFRED  MARK  WEBB,  F.L.S., 

Editor  of  'Knowledge  '  and  Honorary  General  Secretary  of  the 

Selborne  Society. 

Year  by  year  the  interest  which  is  taken  in  the  world  around 
us,  in  the  unspoiled  works  of  Nature,  continues  to  increase.  It 
is  now  also  generally  recognized  that  to  train  the  powers  of 
observation  is  one  of  the  most  important  necessities  in  general 
education,  and  that  it  is  far  better  for  everyone  to  teach  them- 
selves naturally  through  the  interest  aroused  by  the  subjects 
considered,  than  to  learn  nothing  but  second-hand  facts  from 
others. 

To  the  majority  of  people,  whether  they  are  children  or  not, 
living  things  prove  most  attractive.  The  general  appreciation 
of  them  begins  most  easily  and  properly  out  of  doors,  and  may 
continue  as  a  lifelong  pursuit.  The  detailed  investigation  of 
some  part  of  natural  history  forms  an  interesting  hobby  as  well 
as  a  healthy  form  of  exercise,  recreation,  and  relaxation,  par- 
ticularly for  those  who  are  not  forced  to  spend  all  their  spare 
time  upon  games,  or  to  whom  the  more  violent  forms  of  athletics 
and  the  usual  kinds  of  sport  do  not  appeal. 

Whatever  line  of  study  is  taken  up,  it  will  be  very  soon  found 
that  if  any  real  progress  is  to  be  made — if  anything  new  is  to  be 
found  out  with  regard  to  the  structure  of  the  various  creatures 
apart  from,  the  obvious — some  aid  to  the  vision  must  be  sought, 
some  means  of  learning  details  which  cannot  be  seen  with  the 
naked  eye.  Here  it  is,  then,  that  the  microscope  comes  into 
play,  and  it  is  not  too  much  to  claim  that  besides  being  the 
source  of  additional  interest,  the  instrument  is  a  great  educator 

292 


THE   MICROSCOPE  AND  NATURE  STUDY         293 

— that  is  to  say,  it  trains,  without  appreciable  effort,  the  hand  to 
be  skilful,  the  eye  to  appreciate,  and  the  brain  to  elucidate. 
Moreover,  without  for  one  moment  suggesting  that  observations 
in  the  open  air  should  not  be  considered  the  most  essential  part 
of  Nature  study,  we  must  agree  with  a  recent  writer  in  THe 
Country  Home,  that  '  there  will  be  times  when  the  most  en- 
thusiastic Nature  student  cannot  be  out  of  doors — long  dark 
winter  evenings  and  wet  days  even  in  summer,  when  indoor 
work  must  take  the  place  of  outdoor.  It  is  then  that  work  with 
the  microscope  will  prove  such  a  fascinating  hobby,  supplement- 
ing, as  it  does,  the  observations  made  with  the  naked  eye,  and 
leading  us  into  regions  where  it  is  impossible  to  travel  with- 
out it.' 

The  lowest  forms  of  plant-life  are  unicellular,  and  often  ex- 
tremely small.  The  microscope  reveals  to  us  that  they  have 
powers  of  locomotion  ;  it  shows  us  also,  for  instance,  that  the 
green  colouring  on  trees  and  fences  which  shows  after  rain  is 
made  up  of  myriads  of  minute  plants,  taking  in  gases  from  the 
air  and  earth-salts  from  the  surfaces  on  which  they  live,  and 
making  their  food  in  the  same  way  as  the  cabbage  or  the  oak- 
tree.  The  whole  science  of  bacteriology  and  the  discoveries  of 
the  minute  fungi  which  cause  disease  and  putrefaction,  which 
give  the  taste  to  butter  and  the  flavour  to  cheese,  entirely 
depends  upon  the  high  powers  of  the  microscope,  and  though 
investigations  in  this  direction  are  not  for  the  young  beginner, 
still,  they  lie  before  him  when  he  has  mastered  the  details  of 
his  instrument. 

The  story  of  the  interesting,  though  for  a  long  time  hidden, 
methods  of  reproduction  among  the  mosses  and  ferns  have  been 
revealed  by  the  microscope.  The  eggs  and  motile  fertilizing 
bodies  in  the  so-called  flowering  heads  of  the  moss  have  been 
discovered,  and  the  determination  of  the  species  by  the  syste- 
matist  depends  to  a  great  extent  upon  the  microscopic  details  of 
the  capsules  which  grow  from  the  egg,  and  are  really  another 
generation,  producing  spores  without  fertilization  and  getting  its 
nourishment  from  the  original  moss-plant.  In  the  ferns  and 
their  allies,  however,  we  find  the  sexual  organs  are  borne  by  a 
tiny  plant  like  a  minute  liverwort,  which  springs  from  the  spores 
on  the  fern  frond  when  they  fall  to  the  ground,  and  this  little 


294  MODE  EN  MICROSCOPY 

plant  nourishes  the  egg  as  it  develops  into  a  new  fern,  which, 
unlike  the  green  moss-plant,  is  the  sexual  generation. 

Before  going  on  to  speak  of  the  use  of  the  microscope  in  the 
various  branches  of  natural  history,  we  may  point  out  that  it 
can  with  advantage  be  used  from  time  to  time  to  lend  an 
additional  interest  to  ordinary  Nature  study. 

The  youngster  who  sees  the  pollen  of  various  kinds  of  flowers 
as  a  powder  may  well  be  introduced  to  the  variety  of  shapes 
and  sculpturing  which  the  grains  present.  Many  of  the  hairs 
which  clothe  and  protect  the  commonest  plants  are  fascinating 
when  their  details  can  be  seen. 

An  unfortunate  occurrence  such  as  the  stinging  of  a  youthful 
naturalist  by  wasp  or  bee  may  well  lead  to  the  examination  of 
the  sting  of  the  insect,  and  possibly  the  hairs  of  the  nettle,  while 
the  delicacy  of  natural  objects  compared  with  those  made  by 
man  may  well  be  brought  home  by  examining  the  point  of  a  fine 
needle  with  a  microscope,  and  seeing  how  far  more  clumsy  it  is 
than  either  of  the  other  two  structures  to  which  allusion  has 
been  made. 

We  need  not  deal  further  with  this  side  of  the  question,  for 
those  who  look  for  suggestions  as  to  microscopic  work,  can 
glean  them  from  the  paragraphs  in  which  more  systematic  work  is 
discussed.  It  may  be  pointed  out  here,  however,  that  the  micro- 
scope may  be  used  by  the  young  student  so  soon  as  the  informal 
stage  of  Nature  study  is  passed.  The  writer  can  say  from  personal 
experience  that  the  interest  which  can  be  aroused  is  very  great, 
while  excellent  work  with  the  microscope  has  been  done,  for 
instance,  by  gardening  lads  who  for  years  have  used  no  instru- 
ments of  greater  precision  than  spades,  and  rakes,  and  hoes. 

We  need  not  dwell  any  longer  upon  the  botanical  side,  except 
to  say  that  the  whole  structure  of  plant  bodies  lies  before  the 
student.  There  are  all  the  interesting  details  to  be  worked  out 
in  connection  with  the  formation  and  storage  of  starch  grains, 
which  vary  in  different  plants ;  while  one  may  examine  the 
delicate  hairs  on  the  roots  which  take  up  water  and  food 
materials,  or  the  thread-like  fungi  which  sometimes  enter  into 
partnership  with  the  roots  and  do  the  work  of  root-hairs,  as  in 
the  heaths  and  rhododendrons,  not  to  mention  the  bacteria-like 
organisms  which  produce  nodules  on  the  roots  of  plants  belonging 


THE  MICKOSCOPE  AND  NATUEE  STUDY         295 

to  the  pea  family,  and  supply  their  willing  or  unwilling  hosts 
with  nitrogen  which  they  are  able  to  get  from  the  atmosphere. 

Then  there  is  the  structure  of  the  fibres  which  prevent  stems  from 
breaking,  and  of  the  tubes  which  carry  water  with  all  the  various 
kinds  of  thickenings  which  strengthen  their  walls  and  prevent 
them  from  being  crushed  in  as  the  plant  grows  and  the  pressure 
within  the  bark  increases.  There  is  the  bark  itself,  made  up  of 
many  empty  brick-shaped  cells,  and  the  places  known  to  botanists 
as  lenticels,  where  the  bricks  are,  as  it  were,  heaped  together, 
instead  of  making  a  solid  wall,  so  that  air  can  penetrate  to  the 
living  tissues  below.  With  the  microscope  we  see  how  the 
annual  rings  come  to  be  made  in  a  woody  stem.  We  can  learn 
the  structure  of  a  bud  and  the  growing  tip,  and  really  come  to 
know  how  a  plant  is  built  up. 

If  the  living  plants  should  pall,  we  can  cut  thin  slices  of  fossils 
and  trace  the  affinities  between  plants  of  bygone  times  and  those 
of  the  present  day.  In  fact,  there  is  no  end  to  the  beauty  and 
the  interest  that  is  revealed  by  the  microscope  when  it  is  brought 
to  bear  on  plant  structures. 

If  the  material  offered  by  the  animal  world  is  not  more  varied, 
it  is,  if  possible,  even  more  attractive  than  that  which  is  to  be 
looked  for  among  plants.  To  be  sure,  the  botanists  have  the 
beautiful  flinty  shells  of  the  diatoms  to  study,  but  among  the 
unicellular  animals  there  is  a  wealth  of  forms  which  are  pro- 
vided with  calcareous  shells  of  many  shapes,  or  which  build 
them  up  with  the  marvellous  discrimination  which  may  exist 
even  in  a  microscopic  speck  of  protoplasm,  from  sand  grains,  or 
the  flinty  needles  of  sponges.  It  is  these  shells  of  foraminifera 
which,  to  a  large  extent,  form  the  ooze  which  is  taken  from  the 
very  greatest  depths  of  the  ocean. 

Other  slightly  higher  forms  have  internal  silicious  skeletons, 
and  may  be  caught  living  in  fine  nets  or  their  skeletons  obtained 
from  deposits  such  as  the  Barbados  earths,  which  are  largely 
composed  of  them. 

There  are  many  of  the  creatures,  such  as  the  bell  animalcule, 
the  slipper  animalcule,  and  any  number  of  other  Infusoria  whose 
conformation  appeals  to  the  eye  and  whose  life-histories  are 
fraught  with  interest.  In  the  sponges  our  microscope  tells  us 
that  amongst  the  supple  horny  fibres  or  the  delicate  needles 


296  MODERN  MICROSCOPY 

and  geometrically  formed  spicules  which  make  up  the  skeleton 
there  are  small  chambers  in  which  lie  the  working  cells  that 
differ  in  practically  no  respect  from  Infusoria.  A  step,  however, 
takes  us  to  the  polyps,  and  a  favourite  subject  for  study  is  the 
little  fresh-water  form,  with  its  waving  arms,  covered  with  sting- 
ing thread-cells  that  aid  it  to  obtain  its  prey.  Its  body,  which 
is  all  stomach  as  it  were,  is  lined  with  cells  resembling  the 
proteus  animalcules  of  the  ponds,  but,  unlike  them,  unendowed 
with  the  power  of  individual  locomotion. 

For  charm  of  shape  and  delicacy  of  construction  commend  us 
to  the  skeletons  of  the  polyps,  which  live  in  colonies,  to  the  fixed 
growths  on  rocks  and  sea-shells  which  at  first  sight  look  like 
delicate  seaweed,  but  which  on  examination  are  seen  to  bear 
innumerable  little  cups  in  which  the  polyps  are  seated.  Almost 
microscopic,  too,  are  the  small  jelty-fish  which  bud  off  from  these 
colonies,  and  by  an  alternation  of  generations  reproduce  not  them- 
selves, but  colonies  like  those  from  which  they  sprang.  Passing 
over  the  hedgehog-skinned  creatures  covered  with  little  nipper- 
like projections  or  spines,  whose  internal  structure  is  of  suffi- 
cient beauty  to  repay  the  trouble  of  grinding  sections,  and 
leaving  on  one  side  the  wheel  animalcules,  pretty  Polyzoa,  and 
the  host  of  creatures  known  as  worms,  which  offer  many  problems 
to  the  biologist,  we  come  to  the  molluscs. 

Their  shells  are  usually  large  enough  to  see  with  the  naked 
eye,  but  there  is  a  great  fascination  about  the  structure  of  their 
calcareous  coverings.  Simple  though  this  may  be  in  the  bivalves, 
it  is  intricate  and  puzzling  enough  in  the  univalves  to  satisfy 
the  demands  of  those  who  wish  to  exercise  their  brains  and 
ingenuity.  Then  the  examination  of  the  tongue-like  organs  of 
the  slugs,  and  whelks,  and  limpets  is  an  aid  to  the  classification 
of  these  forms.  These  structures  themselves,  covered  as  they 
are  with  minute  rasp-like  teeth,  are  so  very  varied  and  beautiful 
that  the  pleasure  of  examining  them,  apart  from  their  scientific 
investigation,  can  be  well  understood.  The  true  snails,  again, 
are  often  provided  with  minute  calcareous  spicules  of  character- 
istic shape  in  the  various  species,  called  the  '  love-darts,'  which 
are  useful  for  classification  purposes,  and  form  beautiful  objects 
for  display. 

It  is  when  we  come  to  the  insects,  however,  that  we  meet  with 


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Fig.  87.— Section  of  Flower-Bud  of  Liey,  showing  Ovary, 
Anthers.   Petals,  Pollen  Grains,   etc. 


Fig.  88.— Lophopus  Crystallinus.  Fig.  89.     Stephanooeros  Eichhornii 

By  permission  of  Messrs.  Flatters,  Milborne,  and  McKechnie,  Ltd. 


. 


THE  MICROSCOPE  AND  NATURE  STUDY         297 

even  more  unlimited  material.  Some  of  them,  like  the  tiny 
fairy-flies, — of  which,  according  to  Mr.  Enock,  five  can  walk 
abreast  through  the  hole  made  in  a  piece  of  paper  with  a  pin- 
point— are  so  small  that  they  have  to  be  examined  under  the 
microscope  in  their  entirety,  while  the  parts  of  other  insects 
show  a  wealth  of  detail,  and  illustrate  in  a  marvellous  way  the 
changes  which  Nature  can  ring  on  a  single  plan.  Take,  for 
instance,  the  mouth-organs  of  the  cockroach.  At  first  sight 
they  little  resemble  those  of  the  bee,  which  sucks  rather  than 
bites,  and  appear  to  have  no  connection  with  the  proboscis  of  the 
butterfly,  intended  merely  for  drinking  up  honey.  On  careful 
examination,  however,  it  can  easily  be  seen  how  the  mouth- 
organs  of  the  two  latter  have  been  modified  from  the  first,  and 
a  similar  comparison  may  be  made  with  the  stylets  of  the  flea, 
the  piercing  organs  of  the  bug,  and  the  lancets  of  the  gnat. 

It  is  only  when  we  examine  the  wonderful  proboscis  of  the  fly 
that  a  real  difficulty  arises.  We  may  mention  also  the  beautiful 
scales  which  give  the  colours  to  butterflies,  and  which  resemble 
those  found  on  the  more  lowly  wingless  insects  known  as  spring- 
tails  and  bristle-tails,  which  have  never  known  what  it  is  to  fly, 
and  have  only  survived  because  when  their  relatives  took  to  an 
aerial  life,  they  were  thrown  out  of  competition  with  them. 

There  is,  indeed,  no  end  to  the  work  which  can  be  done  on 
insects — their  breathing  apparatus  consisting,  as  it  does,  of  a 
series  of  tubes,  from  which  the  air  is  laid  on,  as  it  were,  all  over 
their  bodies  ;  their  beautiful  antennae,  and  the  joints  of  the  legs 
by  which  beetles,  for  instance,  are  recognized,  offer  fields  for 
inquiry  and  objects  of  interest  of  which  the  student  will  never 
tire.  The  dexterous  dissector  will  find  full  scope  for  his  powers 
when  unravelling  the  organs  of  insects,  and  finding  out  how 
these  are  equally  well  adapted  to  the  requirements  in  the  smaller 
forms  of  life  which  possess  them,  as  are  those  of  the  higher 
animals,  whose  general  anatomy  it  needs  no  microscope  to 
elucidate. 

In  the  vertebrates,  as  in  the  case  of  all  living  things,  the 
minute  structure  must  be  learnt  from  the  microscope,  and, 
though  the  sections  are  more  troublesome  to  obtain  than  in  the 
case  of  vegetable  tissues,  there  are  a  host  of  things  that  can  be 
examined  and  worked  out.     Among  them  we  may  mention  the 


298  MODERN  MICROSCOPY 

scales  of  fishes,  the  hairs  of  animals,  the  feathers  of  birds. 
Everyone  can  see  for  himself  how  the  feather  is  built  up;  and 
although  all  the  larger  feathers  are  made  on  the  same  plan,  there 
are  differences  in  detail.  The  various  birds,  indeed,  might  occupy 
the  attention  of  a  lifetime. 

It  is  not  needful  to  dwell  any  further  on  the  question  of  the 
use  of  the  microscope  to  the  student  of  Nature,  or  what  lies  before 
those  who  decide  to  take  advantage  of  it.  In  conclusion,  one 
may  say  that  it  should  be  part  of  everyone's  education  to  learn 
how  to  use  a  microscope,  and  to  have  some  knowledge  of  the 
minute  details  of  the  living  world. 


CHAPTER   XXVII 

THE    MICEOSCOPY    OF    FOODS 
By  CUTHBERT  ANDREWS,  F.E.M.S. 

The  application  of  the  microscope  to  the  examination  of  sub- 
stances in  common  use,  as  of  foodstuffs,  may  be  considered  in 
two  more  or  less  distinct  ways.  In  the  first  the  instrument  is 
used  to  reveal  the  beauties  of  structure  of  the  various  tissues, 
and  in  this  direction  alone  many  interesting  facts  are  brought  to 
light.  But  the  microscope  has  a  more  utilitarian  mission — 
namely,  the  detection  of  adulteration  and  abnormal  conditions. 
This  function  is  by  far  the  more  important,  and  it  is  mainly  in 
this  direction  that  we  propose  to  consider  food-microscopy  in 
these  pages. 

It  is  only  of  recent  years  that  the  microscope  has  attained  its 
present  high  position  in  analytical  work,  but  it  is  now  fully 
understood  that  with  a  modest  equipment  most  important  facts 
may  be  readily  established,  particularly  where  only  a  very  small 
quantity  of  the  substance  in  question  is  available.  In  such  cases 
the  microscope  becomes  even  more  reliable  than  an  ordinary 
chemical  examination,  as  for  the  latter  we  almost  always  need  a 
fair  quantity  of  material  on  which  to  work  ;  whereas  with  the 
microscope  the  minutest  portion  may  be  subjected  to  the  action 
of  reagents,  the  result  being  equally  reliable  with  tests  made  on 
larger  quantities. 

But  in  employing  the  instrument  in  this  detective  capacity 
it  must  be  always  remembered  that,  valuable  as  a  microscopic 
examination  may  be,  it  is  desirable,  where  possible,  that  it  be 
confirmed  and  amplified  by  ordinary  chemical  methods. 

Microscopic  investigation  has  one  great  advantage.  In  the 
hands  of  an  experienced  observer  it  is  quick,  and  often  serves  to 

299 


300  MODERN  MICROSCOPY 

put  one  on  the  right  track,  where  a  chemical  analysis  may  take 
some  time  to  afford  similar  data.  Further,  the  microscope,  like 
the  camera,  '  does  not  lie ' — much,  and,  given  reasonable  care, 
reliance  may  be  placed  on  its  evidence.  For  instance,  if  a 
sample  of  intimately  mixed  starch-grains  were  submitted  to  a 
chemist,  we  think  he  would  not  be  disposed  to  offer  any  opinion 
as  to  their  origin  on  the  information  afforded  by  a  chemical 
examination.  But  a  microscopist  of  experience  in  this  class  of 
work  could,  with  a  fair  degree  of  certainty,  give  the  proportions 
of  the  various  starch-grains  composing  the  sample. 

On  the  other  hand,  a  microscopical  examination  of  a  film  of 
milk  would  be  no  guide  to  the  fat-contents  of  a  specimen ; 
whereas  by  exceedingly  simple  chemical  means  a  figure  accurate 
enough  for  all  practical  purposes  may  be  readily  obtained. 

We  must  therefore,  on  the  whole,  regard  the  microscope  as  an 
invaluable  assistant,  to  go  hand  in  hand  with  other  and  older 
methods.  The  chief  use  of  the  instrument,  as  above  stated,  is 
to  aid  in  the  detection  of  foreign  substances  in  foodstuffs,  and 
where  the  tissues  present  marked  characteristics  the  visual 
examination  is  highly  important.  Further,  the  microscope  is 
paramount  when  an  examination  for  Entozoa  or  other  parasites 
is  to  be  undertaken. 

Within  the  narrow  limits  of  these  pages  we  cannot  pretend  to 
give  an  exhaustive  account  either  of  characteristic  tissues,  their 
adulterants,  or  of  microscopic  methods ;  but  we  hope  that  the 
brief  outline  furnished  may  be  of  assistance  to  the  working 
microscopist,  for  whose  further  guidance  we  append  a  list  of 
standard  works  which  should  be  consulted,  and  to  the  authors 
of  several  of  which  we  have  to  acknowledge  our  indebtedness. 

A  word  as  to  the  choice  of  a  microscope  for  this  particular 
class  of  work  may  be  helpful.  In  practice  we  find  the  '  H ' 
Edinburgh  Student's,  by  Watson,  an  ideal  instrument.  It  has 
every  convenience  for  research  work,  and  is  also  admirably 
suited  for  photo-micrography,  which  latter  is  often  of  great 
assistance  in  systematic  work.  With  a  couple  of  eyepieces  and 
the  1-inch  and  J-inch  objectives  much  may  be  done  ;  while  the 
addition  of  a  2-inch  and  J-inch,  with,  later,  a  —-inch  oil  immer- 
sion, makes  the  outfit  complete  and  efficient.  All  the  photographs 
reproduced  here  were  taken  by  the  writer  with  this  instrument. 


PLATE    III. 


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Fig.  90. — Starch  of  Wheat. 


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Fig.  91.— Starch  of  Kyi:. 


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Fig.  92. — Starch  of  Barley. 


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Fie.    93. — FlLAI   OF    Be  I  111:. 


Fig.  94. — Penicii.lium  Glatjcum. 


Fig.  95.-Chek.se  Mitj 

[To, 


THE  MICROSCOPY  OF  FOODS  301 

As  a  lower-priced  alternative  to  this  stand  we  favour  the 
'  Fram,'  by  the  same  makers.  Having  no  mechanical  stage,  this 
is  less  convenient  than  the  '  H  ';  but,  after  all,  such  a  stage  is 
largely  a  matter  of  habit,  and  readers  will  find  the  '  Pram  ' 
excellent  and  reliable  in  every  way. 

We  may  now  proceed  to  a  consideration  in  detail  of  some  of 
the  more  important  foods. 

Starches. — Starch  is  the  carbohydrate  content  of  vegetable 
cells,  and  is  found  in  the  majority  of  plants.  It  occurs  in 
various  parts  of  the  growth.  To  the  human  body  starch  is  of 
considerable  value  as  an  energizing  food,  and  in  one  guise  or 
another  it  forms  a  large  percentage  of  the  nutriment  we  take. 
Immediately  on  entering  the  body  it  undergoes  a  change,  being 
converted  into  sugar  by  the  action  of  the  saliva,  and  is  thus 
prepared  for  its  ultimate  digestion  and  assimilation.  The  study 
of  starches  is  therefore  a  very  important  part  of  food-microscopy, 
and  considerable  training  is  necessary  in  order  to  be  able  to 
state  with  certainty  the  origin  of  an  unnamed  starch. 

Continued  research  has  shown  that  starches  possess  some 
very  decided  characteristics.  Thus,  although  different  varieties 
vary  considerably  in  size  and  apparent  structure,  yet  the  grains 
of  a  given  starch  are  remarkably  constant  in  their  appearance, 
or,  alternatively,  show  variation  within  certain  well-defined 
limits. 

The  simplest  method  of  examining  starch  under  the  microscope 
is  to  take  the  smallest  possible  quantity  and  place  on  it  a  drop 
of  distilled  water,  then  applying  a  thin  cover-glass.  Such  films 
are,  of  course,  only  temporary ;  good  permanent  mounts  of 
starch-grains  are  very  difficult  to  prepare. 

Viewed  by  transmitted  light,  a  film  of  starch  presents  a  field 
covered  with  round,  oval,  or  slightly  irregular  bodies,  devoid  of 
colour,  and  highly  refractile.  There  are  usually  to  be  seen  both 
large  and  small  grains,  excepting  in  the  case  of  certain  varieties, 
where  the  individual  cells  are  remarkably  uniform  in  size. 
Some  starches  exhibit  concentric  striations  or  markings,  which 
may  occur  either  with  or  without  a  '  spot '  or  hilum,  the  latter 
being  either  central  or  excentric.  Alternatively,  a  grain  may 
show   no    visible    hilum    or    striation.      These    markings    are 


302  MODEKN  MICEOSCOPY 

obviously  one  method  of  distinguishing  certain  starches  from 
some  others. 

An  examination  should  also  be  made  by  polarized  light,  using 
a  selenite.  By  this  means  the  markings  are  brought  more 
strongly  into  view,  and  other  phenomena,  such  as  the  '  cross  ' 
on  various  grains,  aid  in  the  identification. 

The  most  important  starch  from  a  dietetic  standpoint  is  that 
of  wheat  (Fig.  90).  The  grains  are  large,  round  or  oval,  and  do 
not  exhibit  the  markings  above  mentioned.  There  are  also 
present  numbers  of  small  grains  scattered  through  the  field, 
but  sizes  intermediate  between  the  two  are  rare.  Wheat-starch 
may  thus  be  distinguished  from  rye  (Fig.  91),  which  is  similar, 
but  which  commonly  has  many  grains  showing  deep-rayed 
clefts  and  ragged,  broken  edges  ;  and  from  barley  (Fig.  92), 
which  it  again  resembles,  but  the  grains  of  which  are  rather 
smaller  and  not  so  uniformly  round. 

Bread  consists  mainly  of  starch,  and  if  a  small  fragment  of  a 
white  loaf  be  taken,  and  a  drop  of  the  iodine  solution  (p.  317) 
applied,  a  blue- black  coloration  will  take  place.  This  reaction  is 
peculiar  to  all  starches,  and  is  a  sure  means  of  demonstrating 
their  presence.  It  is  also  of  value  in  examining  '  pre-digested  ' 
breads  and  cereal  foods,  as  the  proportion  of  unchanged  starch 
may  be  estimated  with  a  little  experiment.  Obviously,  the 
larger  the  amount  of  starch,  the  deeper  and  more  decided  the 
colouring. 

In  most  flours  small  fragments  of  bran  are  to  be  found, 
and  these  particles  will  be  readily  noticed  under  a  moderate 
power. 

Flour  (and,  of  course,  bread)  usually  contains  also  some 
proportion  of  aleurone,  the  proteid  substance  of  plants.  This 
will  be  differentiated  by  the  iodine  test  above  mentioned  ;  for, 
while  the  starch  assumes  a  bluish  tinge  under  the  microscope, 
the  aleurone  grains  take  on  a  yellowish-brown  colouring.  A 
confirmatory  method  of  identifying  this  and  other  proteid  sub- 
stances is  by  the  application  of  Millon's  fluid  (see  p.  317),  with 
which  the  proteids  give  a  '  brick-red '  reaction. 

A  further  test  of  bread  should  be  for  acidity.  This  is  indicated 
by  a  solution  of  litmus,  a  drop  or  two  of  which  should  be  added 
to  the  specimen.     If  the  latter  is  acid,  the  liquid  turns  red  ;  if 


THE  MICROSCOPY  OF  FOODS  303 

alkaline,  blue  ;  while  a  neutral  sample  leaves  the  iluid  unchanged. 
Good  bread  should  show  no  acidity,  even  after  keeping  for  several 
days. 

In  the  case  of  '  brown  '  or  wholemeal  breads,  a  lesser  proportion 
of  starch  will  be  found,  with  a  corresponding  increase  in  the 
amount  of  proteid  and  bran. 

The  adulteration  of  white  flours  would  probably  take  the  form 
of  an  addition  of  starches  other  than  that  of  the  grain  repre- 
sented. Some,  such  as  potato,  would  be  at  once  conspicuous,  by 
reason  of  the  marked  contrast  in  size.  Other  mixtures  would  be 
much  more  difficult  to  detect ;  and  in  the  case  of,  say,  a  wheat- 
flour  adulterated  with  barley  or  rye,  it  might  be  necessary  to 
measure  a  large  number  of  grains,  and  to  compare  the  average 
of  these  with  that  of  a  specimen  of  known  purity. 

Alum  has  been  used  as  a  preservative  in  bread,  to  which  it 
also  imparts  a  whiter  appearance,  but  it  is  probably  seldom 
employed  now. 

Butter.  —  Under  ordinary  transmitted  light,  pure  butter 
normally  presents  a  nearly  homogeneous,  colourless  background 
(fat),  with  the  water-globules  standing  out  prominently  in  the 
field  (Fig.  93).  These  globules  should  show  but  little  variation 
in  size. 

Viewed  by  polarized  light,  the  field  should  appear  uniformly 
dark,  and  a  better  examination  may  be  made  if  all  top  light  is 
screened  off  from  the  surface  of  the  slide.  The  presence  of  salt 
or  salicylic  acid  (occurring  as  preservatives)  may  occasion  bright 
appearances,  but  these  will  be  identified  on  examination  by 
direct  light,  by  reason  of  their  refractile  nature. 

In  the  case  of  margarine,  or  foreign  fats  mixed  with  genuine 
butter,  a  more  granular  field  is  seen,  and  large  water-globules 
are  often  present.  Further,  the  polariscope  will  probably  show- 
many  bright  crystalline  points ;  these  latter  are  also  observable 
in  samples  of  rancid  butter,  or  butter  which  has  been  reworked 
or  '  renovated.'  The  number  and  size  of  the  water-globules  also 
vary  with  different  samples,  and  may  be  an  indication  of  the 
origin  and  method  of  manufacture. 

Butter  does  not  commonly  contain  substances  other  than  fat 
and  water,  and,  if  present,  such  additions  would  be  detected 
without  much  difficulty.    Colouring  matter,  and  gross  adulterants 


304  MODERN  MICROSCOPY 

such  as  starch,  would  stand  little  chance  of  being  passed  over 
even  by  a  cursory  examination. 

To  view  butter  conveniently,  a  very  small  fragment  should  be 
placed  on  a  slip,  and  covered  with  a  drop  of  pure  olive  oil.  A 
thin  cover  should  be  very  carefully  pressed  down  on  this,  and  a 
workable  film,  capable  of  being  used  under  a  J-inch,  will  be  thus 
formed. 

The  very  greatest  care  must  be  exercised  in  dealing  with 
butter  samples.  The  art  of  '  blending  '  and  '  faking  '  has  reached 
a  high  pitch,  and  very  scientific  methods  are  now  employed  by 
some  manufacturers.  The  size  and  arrangement  of  the  water- 
globules  ;  the  presence  of,  or  freedom  from,  salt ;  the  degree  of 
freshness  of  the  sample,  and  many  other  conditions,  will  affect 
the  microscopical  appearance  ;  and  it  is  possible  for  a  butter  to 
contain  a  very  high  percentage  of  fat  other  than  milk-fat,  and 
for  this  to  be  still  undetected  by  the  microscope. 

Cheese.  —  Cheese  is  not  very  distinctive  when  examined 
microscopically,  the  most  important  substance  detectable  being 
starch,  which  may  occur  as  an  adulterant.  Very  thin  slices  of 
the  cheese  should  be  cut,  and  the  fat  dissolved  out  with  ether. 
The  starch  and  proteid  may  then  be  demonstrated  by  the  iodine 
and  Millon's  tests  respectively. 

In  old  cheeses  the  various  moulds  and  parasites  may  be 
identified — Aspergillus  glaucus  (the  usual  'blue  mould'),  Peni- 
cillium  glaucum  (Fig.  94),  etc.,  and  the  common  cheese-mite, 
Acarus  domesticus  (Fig.  95).  The  rinds  of  some  cheeses  are 
interesting,  but,  as  this  part  is  seldom  eaten,  they  are  not  very 
important. 

Milk. — It  has  been  stated  already  in  these  pages  that  the 
microscopic  examination  of  milk  is  of  small  service  in  deter- 
mining whether  the  sample  be  '  good  '  or  '  poor  ' — that  is  to  say, 
whether  the  percentage  of  fat  is  up  to  the  normal  standard. 
Certainly,  a  grossly  diluted  sample  might  present  a  field  in  which 
the  fat- globules  were  noticeably  few,  but  in  such  a  case  a  simple 
physical  inspection  would  reveal  the  lack  of  quality  equally  well. 
In  any  case,  such  a  test  would  be  quite  insufficient  in  itself,  and 
the  estimation  of  '  fatty  solids '  must  therefore  be  referred  to 
chemical  analysis. 

But  when  we  come  to  inspect  the  nature  of  any  sediment 


THE  MICROSCOPY  OF  FOODS  305 

which  may  be  thrown  down  in  milk,  we  find  the  microscope  of 
unique  service.  Needless  to  say,  if  milk  is  properly  dealt  with 
in  the  dairy  and  during  distribution,  the  deposit  should  be 
practically  nil ;  but  as  the  desired  care  is  frequently  not  exercised 
by  the  vendor,  it  is  always  advisable  to  make  an  examination, 
which  may  be  on  the  following  lines.  The  milk  should  be  either 
centrifuged  or  allowed  to  settle  in  a  conical  glass,  the  drop  taken 
from  the  point  of  the  vessel,  and  covered  with  a  thin  cover,  as 
suggested  for  the  examination  of  water  (p.  307).  A  film  suitable 
for  viewing  with  a  high  power  may  be  easily  obtained. 

The  microscopic  appearance  of  normal  milk  is  simply  that  of 
an  almost  colourless  liquid,  in  which  may  be  seen  floating  the 
fat-globules.  The  number  of  the  latter  will  be  under  the  normal 
in  a  film  taken  from  a  sample  which  has  been  allowed  to  settle ; 
but  it  is  the  foreign  substances  in  the  field  which  are  of  most 
interest  to  the  microscopist.  Amongst  the  more  important  of 
these  will  be  particles  of  yellowish-brown  vegetable  tissue,  for 
the  presence  of  such  debris  is  a  pretty  sure  indication  of  manurial 
pollution.  This  means  carelessness  on  the  part  of  farm-workers 
and  others  handling  the  milk,  for  which  there  is  no  excuse.  A 
similar  reason  will  usually  explain  the  occurrence  of  grit,  hairs, 
and  fibres,  and  sometimes  fragments  of  hay  and  pollen-grains. 
All  these  things  indicate  the  possibility  of  the  germs  of  disease 
being  introduced  into  the  milk ;  and  as,  with  reasonable  care,  it 
is  possible  to  avoid  running  this  risk,  a  supply  of  milk  which 
presents  these  danger-signals  should  be  vetoed  as  human  food. 

Of  matters  other  than  those  of  vegetable  origin  the  most 
significant  are  corpuscles  of  blood  and  pus.  These  are  found  in 
the  milk  of  cows  suffering  from  tubercle  or  from  an  inflam- 
matory condition  of  the  udder  or  ducts.  The  blood-discs  of  the 
cow  closely  resemble  those  of  man,  being  only  slightly  less  in 
size.  The  pus  cells,  if  derived  from  an  inflamed  udder,  or  more 
remote  locality,  may  be  infiltrated  with  milk-fat,  and  in  this 
state  closely  resemble  colostrum  corpuscles. 

These  last-mentioned  bodies  are  found  in  the  milk  of  cows 

which  have  recently  calved.     They  are  large,  round  masses,  of  a 

granular  appearance,  somewhat  resembling  a  mulberry,  and  are 

believed  to  be  really  white  blood-corpuscles  (leucocytes)  infiltrated 

with  milk-fat.     They  are  coloured  very  slowly  with  aniline  red. 

20 


306  MODERN  MICROSCOPY 

As  the  milk  from  cows  which  have  newly  calved  should  not  be 
taken  for  human  consumption,  it  is  obvious  that  the  presence  of 
colostrum  bodies  again  means  carelessness  or  disregard  on  the 
part  of  the  farmer. 

The  detection  of  preservatives  is  a  fairly  simple  matter,  but 
as  this  is  best  done  with  a  larger  sample  of  the  milk,  it  scarcely 
comes  within  the  range  of  our  inquiries.  Many  of  the  customary 
tests  could,  however,  doubtless  be  applied  to  microscopical 
quantities  if  necessary. 

The  bacteriological  examination  of  milk  is  a  very  important 
study,  and  one  which  is  daily  becoming  more  imperative.  No 
doubt  exists  as  to  the  danger  of  milk  as  a  carrier  of  disease  ;  and 
this  is  readily  understood  when  we  realize  how  well  suited  the 
product  is  to  the  rapid  multiplication  of  micro-organisms.  Films 
may  be  prepared  from  the  sediment,  and  stained  for  tubercle 
bacillus  as  directed  by  Mr.  Cole,  and,  if  necessary,  cultivations 
should  be  made  on  a  suitable  medium.  The  germs  of  typhoid, 
scarlet  fever,  cholera,  and  diphtheria  may  also  be  present,  and 
a  textbook  on  bacteriology  should  be  consulted  for  the  best  way 
of  searching  for  these. 

Fig.  96  illustrates  the  fat-globules  in  a  film  of  pure  milk. 

Water. — Chemically  pure  water  is,  outside  a  laboratory,  an 
impossibility.  Moreover,  it  is  highly  probable  that  such  a  water 
would  be  of  little  or  no  use  as  a  food  ;  for  it  is  not  always 
remembered  that  water  is  needed  in  far  greater  quantity  than 
any  other  foodstuff,  although  this  is  easily  understood  when  we 
recall  the  fact  that  the  body  itself  consists  of,  roughly,  65  per 
cent,  of  moisture,  which  must  of  necessity  be  constantly  replaced 
from  external  sources. 

A  pure  drinking-water  should  contain  little  or  no  sediment, 
and  any  present  should  be  entirely  of  a  mineral  character.  The 
microscopic  examination  of  a  sample  of  tap-water  which  has 
undergone  proper  filtration  will,  in  the  majority  of  cases,  reveal 
little  or  nothing  to  the  observer.  But  where  a  water  has  been 
improperly  purified  or  stored,  many  animal  and  vegetable  sub- 
stances may  be  present,  and  in  the  case  of  some  of  these  a 
source  of  grave  danger  is  indicated.  Particularly  is  this  the 
case  where  the  water  is  drawn  from  a  well,  and  in  considering 
such  samples  notice  should  also  be  taken  of  the  situation  and 


PLATE    IV. 


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Fig.  96.— Film  of  Milk. 


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Fig.  98. — Sewage  Fungus. 


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Fig.  101.     T\  i«n  a  Cells 


THE  MICROSCOPY  OF  FOODS  307 

surroundings.  For  instance,  if  a  manure  heap  should  be  found 
in  fairly  close  proximity  to  the  well,  this  fact  would  probably 
explain  any  indication  of  manurial  refuse  which  might  be 
detected  in  the  water,  and  would  warrant  the  immediate  removal 
of  the  offending  matter. 

We  will  not  take  into  consideration  here  pond  or  other 
open  waters,  as  such  sources  would,  of  course,  yield  unlimited 
organisms  and  foreign  matter  of  various  kinds. 

In  conducting  a  microscopical  examination  of  drinking-water 
it  is  advisable  to  allow  about  a  quart  to  settle  for,  say,  twentv- 
four  hours,  the  top  of  the  container  being  meanwhile  covered  to 
exclude  dust,  etc.  At  the  end  of  this  time  the  bulk  of  the  water 
is  poured  off  or  siphoned  from  the  top  with  as  little  disturbance 
as  possible,  after  which  the  remainder  is  well  stirred  and  con- 
veyed to  a  conical  glass,  in  which  it  should  be  allowed  to  settle, 
as  before,  for  about  twelve  hours.  The  bulk  of  the  water  is 
again  withdrawn,  and  the  sample  for  micro-inspection  taken 
with  a  pipette  from  the  bottom,  or  '  point,'  of  the  conical  glass. 
The  maximum  of  sediment  is  thus  obtained  with  a  minimum  of 
water.  The  drop  is  placed  on  a  slip  under  a  thin  cover-glass 
in  the  usual  way. 

As  before  stated,  a  good  quality  water  will  contain  little 
solid  matter.  Where  pollution  has  taken  place,  however,  the 
variety  of  substances  which  may  be  met  with  is  enormously 
wide.  To  enumerate  them  would  need  much  space,  while  to 
recognize  them  all  at  sight  would  be  beyond  the  power  of 
any  microscopist  we  have  so  far  had  the  honour  of  meeting. 
We  must  therefore  content  ourselves  with  naming  a  few  of 
the  commoner  forms  met  with,  and  would  refer  the  reader 
to  the  various  textbooks  for  a  fuller  list  and  more  extended 
descriptions. 

1.  Inanimate  Matter. — Mineral  substances:  Lime,  clay,  sand, 
etc.,  with  occasionally  traces  of  lead,  zinc,  or  copper. 

Vegetable  substances :  Starch  -  grains,  ducts,  vessels  and 
cuticles  of  plants ;  fragments  of  wood,  roots,  etc. 

Particles  of  wool,  silk,  and  cotton  ;  human  hairs  and  epithe- 
lium ;  muscle  fibres ;  animal  hairs  and  scales,  etc. 

2.  Living  Organisms. — Vegetable:  Bacteria,  diatoms,  desmids, 
Volvox,  Protococcus,  Chara,  Conferva3,  Beggiatoa,  etc. 


308  MODERN  MICROSCOPY 

Animal:  Rotifera,  Vorticella,  Paramecium,  Amoeba,  Euglena, 
tapeworms  and  their  eggs,  etc. 

Mineral  substances  are,  as  a  rule,  readily  recognizable,  and 
have  no  particular  significance.  This  does  not  apply  to  lead, 
zinc,  or  copper,  either  of  which  may  be  present  in  dangerous 
quantities.  The  detection  of  these  metals  properly  belongs  to 
ordinary  chemical  analysis,  as  does  the  estimation  of  ammonia, 
nitrates,  nitrites,  etc. ;  but  a  test  may  with  little  trouble  be  made 
on  the  stage  of  the  microscope. 

Put  a  few  drops  of  the  original  water  into  a  cell,  and  acidulate 
with  a  minute  quantity  of  hydrochloric  acid.  Add  a  drop  or  two 
of  potassium  ferrocyanide.  If  iron  is  present,  a  blue  coloration 
results  ;  if  copper,  the  water  turns  bronze  ;  while  zinc  causes  a 
white  precipitate  to  fall.  For  lead,  test  another  sample  with 
potassium  chromate,  which  gives  a  yellow  reaction. 

Fragments  of  vegetable  origin  are  usually  quite  harmless  in 
themselves,  but  they  suggest  that  the  water-supply  is  open  to 
the  atmosphere. 

The  presence  of  dead  animal  matter  (fibres,  hairs,  etc.)  must 
always  be  looked  upon  with  suspicion.  Although  a  water  con- 
taining such  substances  may  be  in  itself  quite  safe,  yet  it  is 
evident  that  serious  pollution  is  at  least  possible,  and  every 
effort  should  be  made  to  trace  the  source  of  such  a  supply. 

Of  the  living  organisms,  bacteria  are  the  most  significant. 
Where  their  presence  is  suspected,  it  is  desirable  to  make  a 
cultivation  from  the  sediment.  This  is  done  in  petri  dishes  on 
nutrient  gelatine,  and  the  colonies  are  counted  and  estimations 
made  in  the  usual  manner.  Amongst  the  important  organisms 
to  be  looked  for  are  B.  coli  communis  (the  organism  normally 
found  in  the  human  intestine,  but  which  becomes  virulent  in 
typhoid  infection),  and  B.  typhosus  (typhoid  fever).  Many  other 
bacteria  may  be  present,  but  much  training  and  skill  is  necessary 
for  their  detection,  and  a  textbook  on  bacteriology  must  be 
consulted  for  the  details  of  procedure. 

The  presence  of  diatoms  and  other  similar  forms  of  vegetable 
life  does  not  demand  any  special  notice.  While,  however,  most 
of  these  objects  are  practically  without  danger,  some  of  them 
impart  certain  characteristics  to  the  waters  which  they  inhabit. 
For  instance,  according  to  Whipple,  a  fishy  odour  is  given  by 


THE  MICROSCOPY  OF  FOODS  809 

Volvox  (the  beautiful  globe-like  plant  often  found  in  pond-water), 
an  aromatic  odour  by  certain  diatoms  (Meridion,  Tabellari 
and  so  on. 

Fig.  97  shows  a  number  of  the  commoner  forms  of  British 
diatoms. 

Beggiatoa  alba  (Fig.  98)  is  suggestive  of  sewage  contamination, 
the  fungus  being  found  in  considerable  quantities  on  sew;. 
farms.  It  contains  sulphur,  which  accounts  for  the  occasional 
production  in  its  presence  of  an  odour  of  sulphuretted  hydrogen. 
Beggiatoa  forms  masses  of  long  slender  threads  which  are  devoid 
of  colouring  matter  excepting  towards  the  free  extremity,  where 
pigmented  granules  may  be  seen. 

Of  the  animalcule  to  be  found  in  water,  many  forms  will  be 
familiar  to  the  microscopist.  The  possibility  of  sewage  con- 
tamination is  suggested  by  the  presence  of  Amoeba,  Paramoecium 
(Infusoria),  Yorticella,  etc.,  while  the  larger  organisms,  Buch  as 
the  water-fleas,  worms,  etc.,  call  for  no  comment  here. 

If  the  eggs  or  parts  of  Entozoa  (tapeworms)  be  found,  the 
water  should  be  unhesitatingly  condemned  for  drinking  purposes, 
as  these  eggs  are  capable  of  retaining  their  vitality  for  consider- 
able periods,  and  of  developing  on  finding  their  way  to  a  suitable 
host — which,  in  many  cases,  may  be  man.  (See  under  '  Flesh 
Foods.') 

Tea. — This  is  the  dried  leaf  of  Camellia  then.  There  are 
several  species  which  are  cultivated  by  the  grower,  but  tin- 
variation  in  commercial  teas  depends  rather  upon  the  selection 
of  the  leaves  and  the  details  of  preparation  than  upon  any 
botanical  differences.  Nearly  all  teas  as  sold  are  'blended,' 
various  kinds  being  mixed,  so  as  to  secure  a  more  perfect 
result. 

The  tea-leaf  is  best  prepared  for  microscopical  examination  by 
soaking  in  hot  water,  unrolling,  and  placing  between  two  sli; 
As,  however,  such  a  leaf  is  very  opaque,  it  is  desirable  to  employ 
some  means  of  rendering  it  transparent,  and  several  ways  ha 
been  suggested.     Dr.  Wynter  Blyth  proposes  a  simple  method 
follows  :  '  A  portion  of  leaf  is  enclosed  between  two  cover-glasses, 
and  a  weight  (say  a  coin)  placed  on  the  upper  cover.     The  whole 
is  then  heated  with  a  strongly  alkaline  solution  of  permanganate 
of  potash.     The  action  commences  at  once,  and  the  sulista: 


310  MODERN  MICROSCOPY 

must  be  examined  from  time  to  time  to  see  that  the  oxidation 
does  not  proceed  too  far.  The  solution  attacks  the  colouring 
matter  and  cell  contents  first,  then  the  cell  membranes.  When 
the  action  is  judged  to  be  sufficient — e.g.,  when  the  membranes 
of  the  leaf  only  remain — the  fragment  of  leaf  is  washed  in  water, 
and  treated  with  a  little  strong  hydrochloric  acid,  which  dis- 
solves the  manganese  oxide  which  has  been  formed,  leaving  a 
translucent  white  membrane.  Tea-leaf  thus  treated  is  quite 
different  in  appearance  from  other  leaves.' 

The  same  author  suggests  the  preparation  of  what  he  terms 
a  '  skeleton  ash.'  This  is  obtained  by  enclosing  between  two 
covers  as  above  directed,  and  burning  on  a  metal  sheet  the  leaf 
thus  arranged.  Such  preparations  may  be  preserved  by  cement- 
ing the  edges  of  the  two  covers,  or  by  fusing  them  in  a  flame, 
and  will  be  found  to  be  of  the  greatest  assistance  in  identification. 

The  leaf  of  the  tea-plant  is  elliptical,  having  its  margin 
serrated  almost,  but  not  quite,  up  to  the  stalk.  The  ribs  or 
veins  of  the  leaf  run  from  the  midrib  to  within  a  short  distance 
of  the  margin,  when  they  curve  inwards,  thus  leaving  a  clear 
space  around  the  edge  of  the  leaf.  This  clear  space  is  character- 
istic, not  occurring  in  the  sloe,  willow,  beech,  hartshorn,  or  elder 
— the  leaves  most  employed  when  adulteration  takes  place. 

The  epidermis  should  be  removed  from  the  under  surface  of  a 
leaf,  and  examined  separately  (Fig.  99).  It  will  be  found  to 
contain  numerous  stomata  and  hairs.  The  former  are  present 
in  good  numbers,  are  oval  or  round,  and  are  composed  of  two 
guard  cells.  The  hairs  are  simple  and  pointed,  rather  numerous 
on  young  leaves,  but  less  abundant  on  old.  They  are  about 
1  mm.  in  length,  and  0*015  mm.  in  breadth.  A  vertical 
section  of  the  tea-leaf  exhibits  very  distinctive  idioblasts. 
These  occur  most  frequently  in  the  older  leaves,  but  are  never 
entirely  absent.  They  are  long,  tough,  and  branched,  and  may 
be  shown,  without  sectioning  the  leaf,  by  Moeller's  method  of 
warming  some  fragments  of  leaf  in  a  very  strong  solution  of 
caustic  potash,  and  then  pressing  these  firmly  under  a  cover- 
glass.  The  presence  of  the  idioblasts  is  strong  evidence  of  the 
nature  of  the  leaf. 

Tea  contains  an  alkaloid  principle  called  '  theine,'  which  is 
poisonous    when   taken   in    large   quantities.      Theine   may    be 


PLATE   V 


Fig.  102  —Head  of  Cysticercus. 


Fig.  103.— Tj;k  iii\ a  Spikai  i-. 


Fig.  104.— Oxyuris  Veemicularis. 


*:•     - 

Fig.  105.— Li\  i  r   Fi  dke  oi    8 


x. 


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fe 


Fig.  106.— Bacillus  Anthraci* 


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.     - 

1 

Fig.   107.     Hi  m  ^n   I*i 

OOI)    '  ' 

THE  MICEOSCOPY  OF  FOODS  81] 

simply  demonstrated  by  placing  a  little  tea  on  a  watch-gli         ad 
covering  this  with  a  second  glass  of  the  same  size.     Ti 
then  placed  on  a  water-bath,  or  on  a  gauze  over  a  BunseD  flan 
and  in  about  five  minutes  drops  of  moisture  will  be  seen  on  I 
upper  glass.     In  about  ten  minutes  the  theine,  in  its  chai 
istic  crystallized  form  of  long  microscopic  needles,  may  be  bi 
under  a  low  power;  while  after  a  further  five  minutes  quanl 
of  the  crystals  will  have  been  produced.     As  no  theine  is  vola- 
tilized from  exhausted  leaves,  this  test  is  of  value  in  detectii 
tea  which  has  been  redried. 

Coffee.— Coffee  is  the  dried  seed  of  Caffea  arabica,  with  the 
husk   removed.      This    seed   is   roasted,  during    which    proa 
is   liberated    the    oil — caffeol — to   which    the   aroma   of    colli 
is  due. 

The  main  portion  of  the  coffee-berry  is  composed  of  thick- 
walled  cells  (Fig.  100),  which,  when  cut  in  radial  sections,  present 
a  distinctive  knotted  and  irregular  appearance.  A  thin,  tough 
membrane  covers  the  berry,  and  this  membrane,  fragment.-  of 
which  may  be  found  in  ground  coffee,  contains  the  typical 
spindle-shaped  cells  illustrated  (Fig.  101).  In  the  unroasted 
berry  globules  of  oil  may  be  detected,  but,  as  above  stated, 
these  are  dissipated  on  heating. 

Coffee  is  chiefly  adulterated  by  an  admixture  of  either  chicory, 
various  cereals,  ground  date-stones,  caramel,  etc.,  the  first- 
named  being  the  most  frequently  found. 

If  a  small  quantity  of  roasted  and  ground  coffee  be  sprinkled 
on  the  surface  of  cold  water,  any  chicory  present  will  sink 
rapidly,  while  the  coffee,  by  reason  of  its  oily  nature,  will  float 
for  some  time.     Further,  the  sediment  of  chicory  ie  1, 

while  the  pure  coffee  remains  hard.  Microscopically,  chicory 
differs  widely  from  pure  coffee,  the  soft  tissues  of  the  formi 
abounding  in  spiral  vessels,  which  are  usually  readily  reco  - 
nizable.  The  roasted  substance  of  beans,  wheat,  acorns,  etc., 
which  have  been  used  as  adulterants,  are  quickly  detected 
the  presence  of  the  respective  starches,  which  are  c  loured  by 
the  iodine  test.  Neither  coffee  nor  chicory  give  the  character- 
istic blue  reaction.  The  epidermis  of  date-stones  lias  more  or 
less  irregularly  shaped  oblong  cells,  unevenly  thickened  and 
frequently  pitted  ;  while  caramel  (burnt  BUgar)  may  be  det( 


312  MODERN  MICROSCOPY 

under  a  low  power  by  its  shining  particles  and  its  solubility  in 
hot  water. 

Cocoa. — Commercial  cocoa  is  the  seed  of  Theobroma  cacao, 
powdered  and  subjected  to  various  processes.  The  original 
tissues  are  very  characteristic,  but  the  finely  powdered  product 
usually  offered  for  sale  is  not  easy  to  identify  microscopically ; 
and,  indeed,  a  supplementary  analysis  by  chemical  methods  is 
almost  indispensable. 

If  a  little  powdered  cocoa  is  stirred  in  warm  water,  and  a  drop 
placed  upon  a  slide,  the  starch-grains  may  be  distinguished. 
These  are  not  unlike  rice-starch,  but  differ  essentially  in  that 
the  rice  granules  are  angular,  while  those  of  cocoa  are  rounded. 
There  are  also  present  small  masses  of  tissue  in  which  are 
occasional  dark  brown  cells.  These  contain  the  pigment  known 
as  '  cocoa-red,'  and  are  coloured  red  by  dilute  sulphuric  acid 
and  violet  by  a  solution  of  acetic  acid  and  alcohol  (Fig.  108). 

On  heating  the  cocoa  to  boiling-point,  numbers  of  oil-globules 
separate.  The  starch,  however,  gelatinizes  very  slowly,  and  the 
iodine  reaction  is  also  very  slow. 

To  further  examine,  remove  the  fat  from  some  powdered  cocoa, 
by  shaking  with  alcoholic  ether  at  intervals  for  some  hours ; 
wash  with  alcohol,  and  examine  in  water.  Add  a  little  chloral 
iodine  and  boil  gently.  The  starch  is  now  blue,  and  any  epidermal 
cells  or  crystals  present  are  observable. 

Cocoa  contains  an  alkaloid  termed  'theobromine' which  is  closely 
related  to  caffein  and  theine.  It  sublimes  at  170°  C,  forming 
microscopic  needles.  The  principal  adulterants  are  sugar,  starch, 
and  other  more  harmful  substances  ;  but  the  detection  of  most 
of  these  must  be  referred  to  means  other  than  microscopical. 

Mustard.  —  Mustard  is  the  seed  of  the  black  and  white 
mustard  plants,  Brassica  nigra  and  Sinapis  alba.  In  preparing 
the  condiment  as  usually  sold,  the  husks  of  the  seeds  are 
removed,  and  the  latter  ground  to  a  fine  flour,  which  is  separated 
into  various  grades  by  passing  it  through  sieves.  The  different 
mustards  are  not  usually  sold  in  a  '  pure '  state.  The  commercial 
product  consists  of  a  mixture  of  the  white  and  black  flours,  with 
or  without  wheat-starch,  the  larger  the  proportion  of  black  flour, 
the  stronger  and  more  pungent  being  the  condiment. 

White  mustard-seed  consists  of  an  outer  husk  and  the  seed 


THE  MICROSCOPY  OF  FOODS  :;i im- 

proper.    The  husk  is  of  very  complicated  structure,  being  made 
up  of  six  coats  or  layers,  the  outer  of  which  is  of  a  nmcilagino 
nature  (Fig.  109).     In  the  husk  is  also  found  the  aleurone  la 
— cells  containing  a  proteid  substance  common  to  many  varieties 
of  seeds.     In  ground  mustard,  however,  the  whole  of  the  husk 
has  usually  been  removed,  and  one  of  the  most  valuable  men 
of  identification  is  thus  lost. 

The  substance  of  the  seed  consists  entirely  of  minute  oil- 
bearing  cells.  These  are  very  similar  in  appearance  to  Btarcfa 
granules,  but  they  do  not  give  the  usual  iodine  reaction,  nor  do 
they  polarize  light.  Mustard  may  be  regarded  as  one  of  the 
purest  food  substances  sold.  When  adulteration  takes  place, 
this  is  usually  in  the  nature  of  an  addition  of  wheat  or  other 
flour.  On  the  application  of  the  iodine  test,  any  such  admixture 
is  at  once  revealed,  the  starch  turning  blue.  As  mustard  is 
naturally  entirely  free  from  starch,  the  reaction  above  mentioned 
is  positive  evidence  of  an  added  substance.  Any  aleurone  grains 
present  assume  a  yellowish-brown  tint  under  this  test. 

Mustard  is  sometimes  coloured  by  the  addition  of  turmeric. 
This  is  detected  by  subjecting  the  sample  to  the  action  of 
ammonia,  which  produces  a  brownish-red  colour. 

Pepper. — Both  black  and  white  pepper  consist  of  the  fruit 
or  berry  of  Piper  nigrum,  the  difference  being  that  the  former 
is  the  fruit  in  an  unripe  condition,  while  in  the  latter  it  is 
mature,  and  deprived  of  its  outer  layer.  Unlike  mustard, 
pepper  contains  a  quantity  of  starch,  which  forms  the  central 
mass  of  the  peppercorn.  It  contains  also  about  1  per  cent,  of 
natural  oil. 

If  a  peppercorn  be  sectioned,  it  may  be  easily  identified  with 
the  diagram  given  in  many  textbooks  or  with  a  type  specimen 
to  be  obtained  from  the  usual  sources  (Fig.  110).  The  outermost 
layer  consists  of  hard  cells,  more  or  less  oval  in  shaiu\  and 
having  a  curious  '  pleated  '  appearance ;  while  the  other  tissues 
in  their  sequence  are  the  outer  mesocarp,  fibrovascular  layer, 
oil-cells,  pigment  layer,  hyaline  tissue,  and  the  aleurone  starch 
and  resin  cells. 

A  little  powdered  pepper  (Fig.  Ill)  should  be  diffused  in  a  film 
of  water,  when  the  minute  starch-grains  may  be  identified  by  the 
iodine  test  (p.  317).    Moisten  some  pepper  with  a  drop  of  alcohol, 


314  MODERN  MICROSCOPY 

allow  it  to  stand  for  a  few  moments,  and  then  add  a  drop  of 
dilute  glycerine.  Cover,  and  after  about  five  minutes  the  pre- 
paration may  be  examined.  A  number  of  long,  narrow  prismatic 
crystals  may  now  be  seen ;  these  are  piperine,  one  of  the  alkaloid 
principles  of  pepper.  An  orange-red  coloration  is  imparted  by 
treating  with  concentrated  nitric  acid,  and  if  the  crystals  be 
further  subjected  to  the  action  of  caustic  potash,  a  blood-red 
tint  is  produced. 

The  adulterants  of  pepper  are  legion,  although  probably  not 
many  are  now  employed.  A  number  of  these  are  not  detectable 
by  microscopical  means  alone,  and,  in  any  case,  reference  should 
be  made  to  the  wTorks  on  the  subject  for  a  full  description  of 
minute  structure,  etc. 

Sugar. — The  microscopical  examination  of  sugar  reveals  but 
little  of  value  to  the  observer,  and  it  is  probable  that  this  is  the 
purest  of  all  '  manufactured  '  foodstuffs.  If  a  sample  will  pass 
an  ordinary  physical  examination  it  may  be  fairly  assumed  to  be 
fit  for  consumption.  In  the  case  of  powdered  or  granulated 
sugars,  there  is,  of  course,  always  the  possibility  of  an  accidental 
admixture  of  dust,  debris  from  packing,  etc.,  and  this  is  best 
detected  by  microscopical  means.  Again,  in  the  sugar  known 
as  '  Demerara '  may  sometimes  be  found  the  '  sugar-louse,' 
Lepisma  saccharina,  well  known  to  microscopists  by  reason  of 
the  distinctive  markings  on  its  scales,  which  are  frequently  used 
as  tests. 

Various  sugars  take  characteristic  forms  on  recrystallization, 
and  such  preparations  are  very  beautiful  and  of  value  in  iden- 
tification (Fig.  112).  It  is,  however,  a  difficult  matter  to  obtain 
constant  forms,  as  the  crystals  vary  considerably  according  to 
the  method  of  drying,  etc. 

Flesh  Foods. — The  microscopy  of  flesh  foods  is  in  a  somewhat 
different  category  from  that  of  other  substances  which  we  have 
been  considering  in  these  pages.  Our  efforts  hitherto  have  been 
mainly  directed  towards  a  detection  of  adulteration  ;  but  as  this 
is  not  possible  with  flesh  foods,  at  least  in  the  case  of  fresh  meat, 
we  have  now  to  turn  our  attention  to  the  recognition  of  diseased 
or  otherwise  unhealthy  tissues.  So  far,  of  course,  as  freedom 
from  putrefaction  goes,  our  eyes  and  noses  are  the  best  detec- 
tives ;    but  we  need    something   more  than   this   for  complete 


PLATE    VI. 


,l». 


9 


Fig.  109.— Mustard. 


.^ 


SSsT 


Fig.  110. — Section  of  Peppercorn. 


Fig.  111. — Peppek. 


Fig.  112.— Cane-Sugak  (recrystallizep).        Fig.  113.-  me> 


THE  MICROSCOPY  OF  FOODS  815 

safety,  and  as  a  means  of  discovering  more  obscure  troubh  -  the 
microscope  is  invaluable.  In  fresh  meat  we  have  to  consider 
the  possibility  of  several  dangers  —  namely,  the  presence  of 
animal  parasites,  Entozoa,  etc.;  pathological  condition-  of  the 
flesh  itself — tubercle,  anthrax,  and  other  diseases  due  to  bacteri 
and  the  less  important  addition  of  borax,  nitre,  or  salt,  which 
may  have  been  used  as  a  preservative. 

We  cannot  here  extend  our  inquiry  to  cover  the  composition 
of  canned  or  potted  meats,  sausages  of  various  kinds,  or  other 
forms  of  meat  more  or  less  disguised.  The  analysis  of  these  is 
a  task  much  more  lengthy  and  complex,  for  in  such  cases  we 
have  to  identify,  perhaps,  sundry  dyes  and  adulterants,  besides 
various  parts  of  the  animal  not  usually  eaten  in  a  fresh  condition. 

We  will  therefore  turn  our  attention  first  of  all  to  the  internal 
parasites  or  worms,  usually  included  under  the  term  '  Entozoa.' 
Amongst  the  most  important  of  these  are — 

Cysticercus  celluloses,  the  larval  form  of  Taenia  solium.  These 
are  small  round  or  oval  cysts,  most  frequently  found  in  the 
muscle  of  the  pig,  embedded  in  the  connective  tissue  between  the 
fibres.  The  cyst  has  a  greyish  appearance,  and  is  about  the  size 
of  a  small  bean.  When  the  infected  animal  is  killed,  and  these 
cysts  are  transferred  in  the  meat  to  a  suitable  host — e.g.t  man — 
the  larva  develops  into  the  tapeworm,  Taenia  solium  (Fig.  118  . 
This  worm  is  distinguished  by  its  round  head,  which  is  furnished 
with  four  suckers  and  about  twenty-six  booklets  arranged  in 
the  form  of  a  circle  (Fig.  102).  It  may  reach  a  length  of 
2  metres. 

Cysticercus  bovis,  the  cystic  form  of  Taenia  saginata  {=  T.  medio- 
canellata).  Cysticercus  bovis  is  found  in  the  muscles  of  the  ox, 
cow,  and  calf,  from  which  it  is  transmitted  to  man  when  the 
animal  is  killed  and  eaten  as  food.  It  then  develops  into  the 
tapeworm,  and  may  attain  the  enormous  length  of  4  metr< 
Taenia  saginata  has  a  large,  flattened  head,  furnished  with  four 
suckers,  but  devoid  of  hooks. 

Taenia  echinococcus  :  A  small  tapeworm,  about  5  nun.  in  length, 
developing   especially   in   the   lungs   and    liver   of   herbivore 
animals.     The  eggs  of  this  parasite  are  readily   developed   in 
man,  and  may  produce  fatal  results. 

Trichina  spiralis:    This    is  a  minute   worm   which    is    found 


316  MODERN  MICROSCOPY 

encysted,  most  frequently  in  the  muscle  fibres  of  the  pig.  The 
male  worm  is  about  1J  mm.  in  length,  the  female  3J  to  4  mm., 
and  these  are  found  coiled  up  within  a  calcified  cyst  (Fig.  103).  In 
this  condition  it  is  comparatively  harmless,  and  its  development 
only  takes  place  when  the  host  is  killed.  The  cyst  is  then 
dissolved  by  the  digestive  juices  in  the  new  host,  when  the 
worms  are  free  to  pass  into  the  intestine.  Here  a  more  serious, 
and  even  fatal,  disturbance  may  be  caused  in  man ;  but  if 
the  infection  be  slight,  the  worms  may  pass  to  the  muscles  of 
the  new  host,  again  become  encysted,  and  the  person  suffer 
no  subsequent  discomfort.  In  examining  for  trichinae,  etc., 
sections  should  be  cut  in  the  direction  of  the  muscle  fibres, 
choosing  pieces  near  to  attachment  to  sinew  or  bone.  Prepare 
as  directed  by  Mr.  Cole,  and  examine  with  low  and  medium 
powers. 

Distomum  hepaticum  :  The  liver  fluke  (Fig.  105).  A  large,  flat 
organism  J  inch  to  1  inch  in  length.  It  is  commonly  found  in 
the  livers  of  sheep,  but  may  occur  in  other  animals.  In  the 
fully  developed  stage  the  fluke  is  not  injurious  to  man. 

Tuberculosis. — This  disease  affects  several  of  the  domestic 
animals — the  cow,  and  pig,  and  poultry.  The  tubercles  are 
found  in  various  stages  in  the  lungs,  and  later,  as  the  disease 
gains  a  firmer  hold,  in  the  other  organs  and  the  muscular 
tissues.  Sections  should  be  prepared  in  the  usual  way,  and 
the  bacillus  searched  for.  When  the  infection  is  confined  to 
certain  organs  of  the  animal,  such  as  the  lung  and  lymphatic 
glands,  it  is  not  usual  to  condemn  the  entire  carcass  ;  but  where 
the  disease  has  become  general  and  the  animal  is  emaciated, 
it  must  not  be  used  for  human  food. 

Other  bacteriological  infections  are  septicaemia,  swine  fever, 
tetanus  ('lockjaw'),  cholera,  anthrax  (Fig.  106),  glanders,  etc. 
Where  any  of  these  are  suspected,  pieces  of  the  flesh  should  be 
placed  in  alcohol  to  harden,  and  then  embedded,  cut  and  stained, 
the  organisms  being  then  identified  by  reference  to  works  on  the 
subject. 

If  systematic  work  is  to  be  done  in  food-rnicroscojry  it  is 
suggested  that  a  set  of  type  specimens  of  food  substances  be 
arranged,  to  aid  in  the  identification  of  samples  under  notice. 


THE  MICROSCOPY  OF  FOODS  317 

Most  typical  tissues  may  be  obtained  ready  prepared,  while 
other  substances  may  be  mounted  by  the  worker  himself  as 
opportunity  offers. 

A  nucleus  collection  might  be  on  the  following  lines : 

Water.—  A  full  selection  of  possible  objects  would  be  very  extensive,  and 
it  is  advised  that  such  substances  as  point  to  organic  pollution  are  the  in.,,! 
important  for  our  purpose.  Typical  fibres  of  wool,  cotton,  flax;  epithelial 
cells;  Beggiatoa  alba;  a  slide  of  strewn  diatoms;  Infusoria  (say  Para- 
mcecium) ;  Amoeba. 

Milk.— Colostrum  bodies;  a  preparation  containing  pus  cells,  and  one  of 
blood-discs. 

Cheese. — Aspergillus  glaucus ;  cheese-mites. 

Flour. — Starches  of  wheat,  rye,  and  barley ;  aleurone -grains  ;  wheat- 
grains. 

Tea. — Portion  of  leaf  showing  hairs  and  stomata. 

Coffee. — Section  of  berry ;  ground  coffee  ;  chicory. 

Cocoa. — Pure  cocoa. 

Mustard. — Pure  mustard. 

Pepjyer.  —  Ground  pepper;  section  of  peppercorn. 

Meat. — Section  of  voluntary  muscle ;  fatty  (adipose)  tissue  ;  segment  of 
tapeworm  (Tcenia  solium);  head  of  T.  solium;  head  of  cysticercus  ;  liver 
fluke  ;   Trichina  sjnralis  in  muscle  ;  tubercle. 

The  cost  of  the  above  slides  would  not  exceed,  say,  £2  2s., 
and  they  would  be  found  of  considerable  service. 

The  principal  reagents  for  the  work  are  noted  below.  Most  of 
these  are  best  purchased  ready  prepared,  but  wTe  append  formulae 
for  the  convenience  of  readers  : 

Reagents  for  Starch. — Iodine,  2  grammes  ;  potassium  iodide,  1  gramme; 
distilled  water,  200  grammes.  Make  a  clear  solution.  Stains  starch  blue, 
proteid  yellow  ;  cellulose  yellow,  the  latter  turning  blue  when  treated  witli 
concentrated  sulphuric  acid. 

Reagent  for  Proteid  (Millon's). — Mercury,  3  c.c ;  fuming  nitric  acid. 
27  c.c.  Dissolve  without  heat ;  then  add  an  equal  volume  of  water.  CJivi  s  a 
brick-red  reaction  in  the  presence  of  proteid. 

Chloral  Iodine. — Chloral  hydrate,  50  grammes;  water,  20  c.c.  Dissolve, 
and  then  add  iodine  until  the  solution  is  saturated.  Keep  a  few  crystals  of 
the  iodine  in  the  bottle. 

Osmic  Acid  Solution. — A  1  per  cent,  aqueous  solution  of  osmic  acid. 
Should  be  protected  from  light.     Colours  fat  dark  brown  or  black. 

Litmus  solution.     Sulphuric  acid. 

Distilled  water.     Glycerine.     Alcohol. 


318 


MODERN  MICROSCOPY 


The  following  are  suggested  works  of  reference 


l&o 


'  Foods  :  their  Composition  and  Analysis.'     A.  and  M.  Wynter  Blyth. 

'  Microscopical  Examination  of  Foods  and  Drugs.'     H.  G.  Greenish. 

'  Flesh  Foods.'     C.  Ainsworth  Mitchell. 

•  Practical  Sanitary  Science.'     D.  Sommerville. 

'  A  Compendium  of  Food-Microscopy.'     E.  G.  Clayton. 

The  Microscopy  of  Drinking  Water.'     G.  C.  Whipple. 

The  Microscopic  Examination  of  Drinking  Water.'     J.  D.  Macdonald. 
'  Manual  of  Bacteriology.'     Muir  and  Ritchie. 


INDEX 


Abbe's  apertometer,  64 

camera  lucida,  122 

illuminator,  94 

test-plate,  75 
Aberration,  chromatic,  51 

spherical,  53,  76 
Absolute  alcohol,  142 
Acarus  domesticus,  304 
Accessories,  sundry,  133 
Acetate  of  copper  screen,  92 

of  copper  solution,  189 
Achromatic  t'.apochromatic  objectives,  57 
Achromatism,  50 
Actinomycosis,  171 
Adjusting  a  microscope,  129 
Aleurone,  192 
Algse,  green,  preserving  fluid  for,  189 

marine,  192 

staining  and  mounting,  181 
Alum,  303 

Ammonium  bichromate,  143 
Analyzer,  108 
Anchylostoma,  173 
Andrews,     Cuthbert :     '  Microscopy     of 

Foods,'  299 
Angular  apertures,  62 
Aniline  blue-black,  156 

blue,  156 
Animal  tissues,  hardening  and  preserv- 
ing, 142 
Annular  vessels,  18o 
Antheridia  of  fucus,  191 

of  mosses,  190 
Anthers  of  flowers,  mounting,  177 
Apertometer,  Abbe's,  64 

R.  and  J.  Beck's,  66 

Cheshire's,  66 
Apertometers,  64-66 
Aperture,  numerical,  63 
Apertures  of  objectives,  62 
Aplanatic  aperture,  how  ascertained,  97 
of  condensers,  95 
of  sub-stage  condenser,  97 

319 


Aplanatic  magnifiers,  36 
Aplanatism,  51 
Apochromatic  correction,  51 

v.  achromatic  objectives,  ."7 
Aquaria  for  Rotifera,  250 
Aquarium  microscope.  252 
Aqueous  media,  162 
Archegonia  of  mosses,  190 
Arragonite,  interference  figuivs  of,  236 
Ashe's  camera  lucida,  122 

two-speed  fine  adjustment,  26 
Aspergillus  glaucus,  304 
Attachable  mechanical  stages,  13 

B 

Bacillus,  anthrax,  169 

of  enteric  fever,  171 
Bacillus  tuberculosis,  staining  and  mount- 
ing, 168 
Back  lens  of  objective,  102 
Bacteria,  examination  of  living,  106 

preparation  of,  167 
Baker's  D.P.H.  microscope,  9 
Barley,  mounting,  177 

starch,  302 
Bath,  embedding,  117 
Bausch  and  Lomb  objectives,  7'.' 

research  microscope,  BB,  17 
Beale's  camera  lucida,  122 
Beck,  R.  and  J.'s,  apertometer, 
condensers,  95 
Imperial  microscope,  25 
selenites,  109 

two-speed  fine  adjustment,  26 
Beggiatoa  alba,  30!' 
Berlin  blue  watery  solution.  17." 
Bertrand  plate,  228 
Bertrand's  quarter-quartz  plate,  28S 
Biaxial  crystals,  22  1 
Binocular  eyepieces,    87 

microscope  for  Rotifera,  2( 
Greenough,  35,  39 
Stephenson, 
microscopes,  33 


320 


MODERN  MICROSCOPY 


Bismarck  brown  stain,  171 
Blank  eyepiece,  87 
Bleaching  vegetable  sections,  178 
Blood,  double-staining,  162 

mammalian-staining,  162 

stain,  Ehrlich's  triple,  159 
Toison's,  160 
Blood-vessels,  injection  of,  175 
Body    of    microscope,    sectional    view, 
29 

-tube,  28 
Bone,  section  cutting,  194 
Bones,  decalcified,  208 
Books  on  foods,  318 

on  microscopy,  131 
Borax  carmine,  178 
Botanical  sections,  mounting,  177 
staining,  178 

subjects,  reagents  for,  141 
Botterill's  trough,  126 
Bread,  examination  of,  302 
Buds,    flower,    cutting    and    mounting, 

182 
Bull's-eye  condenser,  110 
how  to  use,  112 
Butter,  examination  of,  303 


Cambridge   Company's    rocking    micro- 
tome, 153 
Camera  lucida,  121-123 
Canada  balsam,  mounting  in,  154,  157, 
184 
mounts,  finishing,  217 
Carmine-injecting  mass,  174 
Cathcart's  microtome,  150 
Celloidin  embedding,  150 

infiltration,  151 

to  remove,  151 
Cells,  preparation  of,  215 
Cementing  lenses,  81 
Cementite,  196 
Centring  a  condenser,  99 

how  to  test,  76 
Chara,  branch  of,  190 
Cheese,  examination  of,  304 
Cheshire     '  Introduction  to  Use  of  Petro- 

logical  Microscope,'  219 
Cheshire's  apertometer,  66 
Chloral  iodine,  317 
Choice  of  an  outfit,  131 
Chromatic  aberration,  51 

correction,  77 

over-correction,  51 

under-correction,  51 
Chromic  acid  and  spirit,  143 
Clearing  sections,  155 
Coal  sections,  195 
Coarse  adjustment,  20 
Cochlea,  hardening  methods,  144 


Cocoa,  examination  of,  312 
Coffee,  examination  of,  311 
Cole's  double-staining  method,  180 

microtome,  148 
Collecting-bottle,  247 

-stick,  246 
Colour,  74 

and  diffraction,  133 

scale,  Newton's,  225 
Compensating  eyepieces,  58,  84,  85 

screws  for  movements,  24 
Compressor  for  Rotifera,  255 
Compressors,  127 
Condenser,  aplanatic  aperture  of,  95,  97 

bull's-eye,  110 

choice  of,  104 

Conrady's  method  of  using,  100 

how  to  centre,  99 

how  to  use,  99 

immersion,  102 

in  an  outfit,  133 

Powell  and  Lealand's,  96 

proportionate  aperture  to  objective, 
101 
Condensers,  R.  and  J.  Beck's,  95 

sub -stage,  93 

Swift  and  Son's,  95 

Watson's,  95 
Conrady's  method  of  measuring  apertures 

of  objectives,  66 
Continental  foot,  10 

Convergent   light,  diagram  of  arrange- 
ment for,  235 
examination  in,  234 
Corallines,  193 
Correction,  chromatic,  77 

collar,  directions  for  using,  71 

collars  for  cover-glass,  70 

semi-apochromatic,  53 
Corrosive  sublimate,  144 
Cotton,  206 

Cover-glass,  influence  of,  69 
Critical  image,  101 
Crystals,  mounting,  204 

to  measure  plane  angles  of,  233 
Curvature  of  the  field,  77 
Cuticles,  206 
Cysticercus  bovis,  315 


D 

Dalton-Smith's  method,  179 

Dark-ground  illumination,  103 
stops  for,  97 
illuminators,  105 

Davis's  shutter,  97,  120 

Decalcifying  solution,  143 

Definition,  tests  for,  77 

Definitions,  50 

Dehydration,  151 

Diaphragm,  51 


INDEX 


32] 


Diatomacere,  cleaning  and  mounting,  210 
mounting  in  Canada  balsam,  210 

Diffraction  effects,  134 
spectra,  51,  101 

Diphtheria  bacillus,  169 

Dissecting  microscopes,  35 

Distomum  hepaticum,  316 

Double  refraction.  222 
staining,  156 

Draw-tube,  correction  of  objective  with 
71 

Draw-tubes,  mechanical,  30 

Drugs,  mounting  of,  183 

Dry  mounts,  183 

finishing,  217 

Dusting  a  microscope,  129 

E 

Earland,  A.  :   '  Collecting  and  Preparing 

Foraminifera, '  270 
Echinus  sections,  195 
Ehrlich's  double  stain  for  bacteria,  168 

hsematoxylin,  155 

triple  stain,  159 
Electric  lamps,  117 
Embedding,  147 

bath,  147 

in  carrot,  148 

in  celloidin,  150 

in  gelatine,  152 
Endothelium,  staining,  163 
Entomological  preparations,  198 
Entomostraca,  narcotizingand  mounting, 

262 
Entozoa,  309 

Eosin  for  botanical  sections,  180 
Epidermis  of  leaf,  185 
Epithelium,  mounting,  163 

staining,  163 
Eyepiece  and  cross- wires,  231 

blank,  87 

negative,  52 

positive,  52 
Eyepieces,  83 

binocular,  87 

choice  of,  83 

compensating,  58,  84 

for  outfit,  133 

Holoscopic,  75,  85 

Huyghenian,  75 

Kellner,  86 

powers  of,  83 

projection,  86 

standard  gauges  for,  87 

universal,  85 
Eyeshade,  128 

F 

Farrant's  medium,  151,  166 

mounts,  finishing,  217 
Fedorow  mica-steps,  228 


Fern,  prothallus  of,  19] 

Ferns,  sporangia  and  Bpores,  10] 

Ferrite,  196 

Field,  curvature  of,  77 

Filaria,  171 
Filters,  coloured,  91 
Finders,  14,  If, 
Fine  adjustment,  22 

to  sub-stage,  1 3 

adjustments,  two-speed,  26 
Finishing  slides,  217 
Fish  scales,  206 
Fixing  and  staining  on  slide,  1 
Flatness  of  field,  74 
Flax,  206 

Flemming's  stain,  158 
Flesh  foods,  examination  of,  31  I 
Flour,  examination  of,  302 
Flower-buds,  preparation  of.  l.Vj 
Fluke  from  sheep,  17 'J 
Focussing  arrangements,  20 
Food,  standard  micro-specimens,  317 
Foods,  microscopy  of,  299 

standard  specimens  for,  316 

reference  works  on,  318 
Foot  of  microscope,  8 
Foraminifera,  cleaning,  271 

and  mounting,  210 

collecting,  272 

'Collecting  and  Preparing,'  by  A 
Earland,  270 

examination  of,  278 

mounting,  280 

sections  of,  280 
Forceps,  stage,  128 
Formaldehyde,  145 
Formalin  for  liverworts,  290 

for  mosses,  290 
Fuchsin,  169 
Fucus,  191 

staining  and  mounting,  181 


G 

Gaffky's  method,  171 

Gas-lamps,  118 

Gauges  for  eyepieces.  s7 

Gelatine  embedding,  1 52 

Geological  sections,  1'.'."' 

Gibbe's  double  stain,  168 

Gifford's  screen,  92 

Glanders  bacillus,  170 

Glands,  digestive,  in  pitcher  plant,  192 

Glycerine  jelly,  U><;.  188 

mounts,  finishing,  217 
Gold  chloride  stain.  162 
Golgi's  nitrate  of  silver  methods,  1  56 
Gram's  method,  169 
Greenough  binocular,  35 
Grenadier's    alcoholic    borax     carmine, 
154 

21 


322 


MODEBN  MICEOSCOPY 


H 

Hematoxylin,  179 

and  eosin,  156 

use  of,  155 
Haematozoa  of  Laveran,  171 
Hairs,  207 
Hardening  reagents,  138 

tissues,  145 
Hausewaldt's  arragonite  figures,  236 
Heliostat,  91 
Hemp,  206 

Holder  for  metallurgical  work,  41 
Holoscopic  eyepiece,  75,  85 

objectives,  Watsons',  57 
Horns,  hoofs,  whalebone,  and  claws,  208 
Horseshoe  foot,  10 
Huyghenian  eyepiece,  75 

eyepieces,  83 
Hydrachnidse,  263 

textbooks  on,  269 


Illuminating  apparatus,  89 
Immersion  condi  nsers,  102 
objectives,  59 

method  of  wiping,  62 
Indian  ink,  method  for  bacteria,  107 
Infiltration  with  paraffin,  147 
Influence  of  cover-glass.  69 
Injecting  apparatus,  175 

masses   175 
Injection  of  bloodvessels,  175 
Insect  mounting  in  glycerine,  200 

preparations,  198 
Insect,  whole,  mounting,  198 

without  pressure,  199 
Iodine  chloral,  317 
Iris  diaphragm,  96 

divisions  to,  97 

nosepiece,  121 


Jackson  foot,  8 

micrometer,  125 

K 

Karyokinetic  stain,  158 
Kellner  eyepieces,  86 
Klein  quartz  plate,  228 
Knowledge,  131 
Kuhne's  method,  170 


Lamps,  electric,  ]17 
gas,  118 
Nernst,  118 
oil,  116 

Swift  and  Son's,  117 


Lard  embedding,  147 
Leaf  hairs,  staining,  181 

scales,  206 

sections,  mounting,  177 
Leprosy  bacillus,  169 
Leitz  inexpensive  microscope,  46 

microscope-stand  F,  23 

objectives,  57 

penological  microscope,  230 
Lenses,  cementing,  81 

polishing,  81 
Light  filters,  91 

polarization  of,  219 
Limbs,  27 

Limestone  sections,  195 
Liq.  potassse,  198 
Live-box  for  Rotifera,  255 

-cages,  126 
Liverworts,  collection  of,  282 

mounting,  285 

preparation  of,  283 
Loffler's  alkaline  blue  method,  169 
Lusgarten's  method,  170 
Lymphatics,  injection  of,  175 
Lymph-sinuses,  injection  of,  176 


M 


Magnification  and  numerical  aperture,  67 
Magnifier  for  Rotifera,  247 
Magnifiers,  37 
aplanatic,  36 
large  lens  and  high  power  impossible, 

37 
Steinheil,  37 
Magnifying   power,    how   obtained    and 

measured,  54 
Maize,  mounting,  177 
Marine  algae,  mounting  process,  192 

glue,  291 
Mayer's  albumen  method,  160 
Measurement  of  objects,  123 
Measuring  numerical  apertures,  64 
Meat,  examination  of,  314 
Mechanical  draw-tube,  30 

movements,  adjustments  for,  24 

order  of  value,  132 
stage,  10 

measurement  of,  124 
stages,  attachable,  13 
long  range,  13 
Metallurgical  microscopes,  40 
Metal  specimens,  195 
etching,  196 
mounting,  197 
treatment  of,  113 
Metals,  holders  for,  41 
Methylated  spirit,  143 
Mica,  224 

quarter-wave  plate,  227,  228 
selenite  stage,  109 


INDEX 


Mica-steps,  Fedorow,  228 
Micrometer,  Jackson,  125 
Micrometers,  Ramsden  screw,  125 

stage  and  eyepiece,  121 
Micro-organisms,  preparing,  167 
Microscope,  adjusting,  129 

care  of,  128 

for  preparing  and  mounting,  46 

packing,  131 

parts  of,  7 

stand,  choice  of,  131 
the,  5 
Microscopes  for  metallurgy,  40 

for  petrology,  44 

for  special  purposes,  35 

portable,  43 

second-hand,  5 

travellers',  43 
Microscopy  of  foods,  299 
Microtome,  Cambridge  rocking,  153 

Cathcart's,  150 

freezing,  149 
Microtomes,  148 
Milk,  examination  of,  304 
Millon's  reagent,  317 
Mirrors,  31 

parallel  worked,  32 

use  of,  32 
Mites,  fresh-water,  263 
collecting,  265 
examination,  267 
mounting,  268 
Mono-bromide  naphthalin  objective,  61 
Monochromatic  blue  light,  effects  of,  93 

light,  89,  102 

apparatus  for,  91 
filters,  91 
Mosses,  190 

'  Mosses    and    Liverworts, '    by    T.     H. 
Russell,   282 

collection  of,  282 

mounting,  285 

preparation  of,  283 
Mounting  in  Canada  balsam,  154,  157 

media,  138 
Miiller's  fluid,  143,  156 
Muscle,  mounting,  164 
Muscular  fibres,  208 
Mustard,  examination  of,  312 
Mvcetozoa,  189 
Myxomycetes,  189 

N 

Narcotizing  Rotifera,  258 

'  Nature  Study,'  by  Wilfred  Mark  Webb, 

222 
Negative  eyepiece,  52 
Nelson's  three-quarter  cone  illumination. 

101 
Nernst  lamps,  118 
Nerve-fibres,  mounting,  165 


Newton's  colour  Bcale,  225 

rings,  81 
Nicol  prism,  its  function,  21 

prisms,  rectangularity  ol  the  rib 

tioli  planes  of,   2 

Nitrate  of  silver  staining,  156 
Nosepiece,  revolving,  11- 
Numerica]  aperture,  63 
and  powei 

increased  by  dense  medium 
apertures,  measuremenl  of, 

O 

Objective  as  condenser,  97 

centring  with  s\\ 

changers,  118 

how  resolving  pow<  r  increased 

observing  hack  Lens  of,  102 
Objectives,  achromatic  v.  apochromatic, 
57 

apertures  of,  62 

apochromatic,  55 

and  high-power  eyepiec 

Bausch  and  Lomb,  79 

care  of,  130 

choice  of,  57,  78 

dry,  water  and  oil  immersion  com- 
pared, 60 

for  outfit,  133 

how  made,  79 

immersion,  59 

method  ol  wiping,  62 

Leitz,  57 

magnifying  power  of,  54 

Reichert,  57 

semi-apochromatic,  ~>1 

Spencer  Lens  Company 

testing,  73 
Oil  immersion  objectives,  59 

lamps,  116 
Olfactory    region,    hardening    methods, 

144 
Oogonia  fucus,  191 
Opaque  mounts,  215 
Optical  construction,  50 

index,  G8 
Osmic  acid,  162 

solution,  317 
Outfit,  choice  of,  131 
Ovaries  of  flowers,  mounting,  177 
Over-correction,  chromatic,   i»*i»ri^_w 

spherieal,  53  rXUrLRTY  £jj 

i      &•  C  Stat*  C 


< 


Packing  a  microscope,  131 

Palates,  207 
Parabolic  reflector,  113 
Paraboloid  condenser,  49 
Paraboloids,  immersion,  106 
Parallin  embedding,  1  17 


324 


MODERN  MICROSCOPY 


Parasites.  201 
Peariite,  197 

Penicillium  glaucum,  304 
Pepper,  examination  of,  313 
Petrological  microscope,  adjustment  of, 
231 
construction  of,  229 
introduction  to  use  of,  219 

microscopes,  44 

optical  adjuncts  for,  227 
Pharmacological  specimens,  183 
Picric  acid,  144 
Picrocarmine  stain,  162 

staining  with,  165 
Pipettes  for  Rotifera,  254 
Pitcher  plant,  192 
Pleurosigma  angulatum,  101 
Polariscope,  108 
Polarization  of  light,  219 

rotary,  226 
Polarizer,  108 
Polishing  lenses,  81 
Pollens,  182 

opaque  mounts  of,  216 
Polycystina,    cleaning    and    mounting, 
210 

mounted  opaque,  211 
Porro  prism  erector,  37,  39 
Portable  microscopes,  42,  43 
Positive  eyepiece,  52 
Potassium  bichromate,  143 
Powell  and  Lealand  condensers,  96 
Preserving  reagents,  138 
Projection  eyepieces,  86 
Proof  plates,  81 
Proteid,  reagents  for,  317 
Prothallus  of  fern,  191 
Protococcus,  190 

Prussian  or  Berlin  blue  injecting  mass, 
174 

Q 

Quartz  cut  parallel  to  axis,  224 

crystals,  223 

wedge,  228 
Quekett  Club  Journal,  131 
Quills,  sections  of,  207 


R 


Ramsden  screw  micrometer,  125 
Raphides,  186,  187,  206 
Reagents,  138 
Refractive  index,  52 

indices,  oil,  water,  and  air,  60 
Reichert  objectives,  57 
Resolving  power,  63 

and  diffraction,  134 
Retardation,  234 

Rocking    microtome,    Cambridge    Com- 
pany's, 153 


Rock  sections,  195 

examination  of,  232 
Root  sections,  mounting,  177 
Rotifera,  237 

apparatus  for  examination  of,  253 
collecting-grounds,  227,  245 
killing  and  fixing,  259 
methods  of  collecting,  245 
monthly  specimens,  219,  237 
mounting,  261 
narcotizing,  258 
preserving  and  mounting,  257 
fluid  for,  260 
Rousselet  on  '  Rotifera,'  219,  237 
Rousselet's  aquarium  microscope,  252 

compressor  and  live-cage,  127 
Royal  Microscopical    Society's    Journal, 

131 
Royal  Microscopical  Society's   standard 

gauges  for  eyepieces,  87 
Russell's    methods    of    collecting,    etc., 

mosses  and  liverworts,  282 
Rye  starch,  302 


S 


Scalariform  vessels,  186 
Scales  of  leaves,  206 
Schutz's  method,  170 
Screws,  female  and  male,  52 
Secondary  spectrum,  52 
Section-cutting,  147 

by  hand,  147 
Selenite,  224 

films,  109 
Semi-apochromatic  correction,  53 

objectives,  57 
Shellac  method,  160 
Side  silver  reflector,  113 
Siedentopf's  ultra-microscope,  48 
Silk,  206 
Silver  crystals,  205 

nitrate,  162 
Soar,   Mr.  C.  D.  :  '  Methods  for  Fresh- 
Water  Mites.'  263 
Spectacles  in  microscopy,  84 
Spectrum,  53 

Spencer  Lens  Company's  objectives.  79 
Spherical  aberration,  53,  76 

over-correction,  53 

zones,  53 
Spicules,  mounting  of,  213 
Spiral  vessels,  186 
Spirillum,  171 
Spirit,  methylated,  143 
Spitta's  apparatus  for  monochromatism, 
91 

'  pot '  green  light  screen,  92 
Sponges,  mounting  of,  213 
Sporangia  and  spores  of  ferns,  191 
Spot  lens,  107 
Stage  forceps,  128 


INDEX 


325 


Stages,  10-16 

Staining  and  fixing  on  slide,  160 

animal  sections,  155 

double,  179 

fluids,  138 

in  bulk,  158 

processes,  154 
Starch,  reagents  for,  317 
Starches,  188,  205 

examination  of,  301 
Steinheil  magnifiers,  37 
Stem  sections,  mounting,  177 
Stephenson  binocular,  35 
Stomata,  185 

Stoney,  Dr.  G.  Johnstone,  heliostat,  91 
Stops,  54 

for  dark-ground  illumination,  103 

Student's  microscope,  7 

Sub-stage,  17 

condensers,  93 

Sugar,  examination  of,  314 

Swift  aud  Son's  condensers,  95 
lamp,  117 

mica-selenite  stage,  109 
pan-aplanatic  objectives,  57 
petrological     microscope,     the 

Dick,  45 
portable  microscope,  42 
Stephenson  binocular,  35 

Swift- Ives  camera  lucida,  123 

Syphilis  bacillus,  170 


Taenia,  172,  315 
Tail  piece  and  mirrors,  31 
Tank  for  Rotifera,  250 
Tape-worms,  172 
Tea,  examination  of,  309 
Teasing  out  tissues,  164 
Testing  objectives,  73 
Tissues,  hardening,  145 
Toison's  stain  for  blood,  160 
Travellers'  microscopes,  43 
Trichina  spiralis,  173,  315 
Triple  stain,  Ehrlich's,  159 
Trough,  Botterill's,  126 
Troughs,  126 

for  Rotifera,  254 
Tube-length,  correcting  by,  71 
Tuberculosis  in  flesh  foods,  316 
Two-speed  fine  adjustments,  26 

U 

Ultra-microscope,  47,  48,  105 
Under-fittings,  19 

-correction,  chromatic,  51 
Universal  eye  pieces,  Swift  and  Son's,  85 


Varnish,  218 

Varnishes,  290 

Vegetable  sections,  bleaching,  178 

mounting,  177 
staining,  178 
Vermes,  172 

Vertical  illuminator,  18,  I  18,  1 1."- 
glare  with,  1 1 .". 

use  of,   11  1 


w 

Water,  examination  of,  306 

immersion  objectivi 
Watson  and  Sons'  holoseop 

85 
Watsons'  fine  adjustment,  -11 
Fram  microscope,  7 
holoscopic  condensers,  : 
objectives,  57 
Webb,  Wilfred  Mark,  on  'Nature Study.' 

212 
Weigert-Pal  method,  158 
Weigert's  method,  171 
Wenham  binocular,  '■'>  I 
Wheat,  mounting,  177 

starch,  302 
White  fibrous  tissue,  mounting,  1 6  1 
Wool,  206 
Wratten  and  Wainwright's  light  lilt 

91 
Wright's  finder,  16 


Yeast,  188 

Yellow  elastic  tissue,  164 


Z 

Zeiss  achromatic  objectives,  56 
aplanatic  magnifier,  247 
apochromatic  objectives,  55 
attachable  mechanical  stage,  15 
binocular  eyepiece,  87 
correction  collar,  70 
dissecting  microscope,  35 
microscope,     Stand     III  , 

through,  12 
objective  changers.  119 
ultra-microscope,  47 

Zeiss's  abbe  apertometer,  65 
camera  bifida,  1 

Ziehl  Neelsen's  stain.  1 

Zones,  spherical. 


8,    HENRIETTA   STREET,    COVENT  GARDEN, 
LONDON,    W.C. 


